G
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Earthworms
Science with Minibeasts: Earthworms
L257
January 2008
Science with
minibeasts:
CLEAPSS
GUIDE L257
Contents
Page
1
Introduction
1.
All about earthworms
2
1.1
Earthworms: Tubes within a tube.
2
1.2
Feeding in earthworms.
3
1.3
Earthworm locomotion.
3
1.4
How does an earthworm breathe?.
4
1.5
Does an earthworm have feelings?.
4
1.6
Regeneration.
5
1.7
Reproduction.
5
1.8
Earthworms and the soil
6
1.9
Worms as food and pollution fighters.
7
First catch your worm
8
2.1
Methods of extracting earthworms.
8
2.2
Collecting extracted worms.
10
2.3
Identifying earthworms
10
Keeping earthworms
11
3.1
Rothamsted wormery.
11
3.2
Alternative viewing wormeries.
12
3.3
A worm box.
12
Investigating earthworms: Outdoor activities
14
4.1
How many worms are there in the soil?.
14
4.2
Investigating casts.
14
4.3
Investigating plugged burrows.
15
4.4
Investigating feeding.
15
4.5
Observing mating.
16
Investigating earthworms: Indoor activities
17
5.1
Explore your worm.
17
5.2
Earthworm movement.
19
5.3
Burrowing.
21
5.4
What do worms eat?.
22
5.5
What can worms sense?.
23
Learning materials & resources
27
2.
3.
4.
5.
6.
Strictly Confidential
Circulate to members and associates only
This guide was originally published by the ILEA Centre for Life Studies. This edition has been revised and
reorganised.
As with all CLEAPSS materials, members and associates are free to copy all or part of this guide for use in
their own establishments.
© CLEAPSS® 2008
CLEAPSS
The Gardiner Building
Brunel Science Park
Uxbridge UB8 3PQ
Tel: 01895 251496
Fax: 01895 814372
E-mail: [email protected]
Web site: www.cleapss.org.uk
Guide L257
Science with minibeasts:
Earthworms
Introduction
“It may be doubted whether there are any other animals
which have played so important a part in the history of the
world as have these lowly creatures.” Charles Darwin, 1881
Darwin’s opinion was based on his personal observations of earthworms, made at Down
House, Kent. He calculated the amount of soil these ‘natural ploughs’ turned over in a year by
weighing surface worm casts. But even his surprising figures were underestimates (because he
didn’t know that most worms don’t leave casts on the soil surface). Darwin calculated that
there were as many as 53,000 (13 per m2) worms in an acre of soil. Rothamsted Research Station
puts the figure nearer one and three quarter million to an acre of fertile farmland (432 per m2)
and a quarter of a million to an acre of poor soil (62 per m2). The contribution of earthworms to
the fertility of soil, through composting and excreting mineral-rich worm casts, is enormous.
And even Darwin overlooked the contribution their burrows make to drainage and aeration.
Earthworms are very common, although seldom seen above ground, and are often easily
collected. They can be the basis for a wide variety of observational and investigatory activities.
They can easily be kept for short or long periods, although they are best housed in cool
temperatures, preferably out of doors.
Earthworms are entirely beneficial animals. The value of earthworms in treating wastes and so
combating pollution, and as a potential source of food (directly or indirectly) for humans, is
now beginning to be realised.
Earthworms belong to a group of invertebrate animals called the annelids which includes all
worms that have segmented bodies. All earthworms belong to the smaller annelid group of the
oligochaetes (meaning ‘having few bristles’); the earthworm’s bristles are very small and not
easily seen but are nevertheless important in movement. Earthworms are closely related to
bristle worms and also leeches.
There are about 1,800 species of earthworms in the world but the commonest worms in Britain
and Europe all belong to one small family - called the lumbricids. Elsewhere in the world, other
families of earthworms are more common. The 28 British species of earthworm all belong to the
lumbricid family, but only about 10 of them are common. All of our earthworms look pretty
much alike, only really varying in size, and to some extent colour. Darwin found it hard to tell
different species apart, so you will be in good company.
Many of Darwin’s studies have never been properly repeated. He often did not distinguish
between different species, and many of the ‘facts’ about earthworms that he established may
not therefore apply to all earthworms. As a result, when earthworms are studied in schools, it
is quite possible that the results will be something completely new to science.
This guide is aimed at encouraging a humane study and appreciation of the earthworm.
1
1. All about earthworms
This section covers the biology of the earthworm.
1.1 Earthworms: Tubes within a tube.
The body of the earthworm is divided into rings or segments. Part way down the animal is a
region that is slightly swollen. This is called the clitellum or ‘saddle’ and it plays an important
part in reproduction. From the outside, there is very little else to see.
The earthworm is basically a long tube with other tubes inside it. You can see this from the
simplified diagram showing the front end of a worm cut open.
‘Heart’
Intestine
‘Brain’
Blood
vessels
Mouth
Nerve cord
The important ‘inner tubes’ are as follows.
The nerve cord. This runs the length of the worm on the lower side of the body. At the front in
the first few segments, it is enlarged as a ‘brain’.
The intestine or gut. This also runs along the whole worm from mouth to anus. It has various
swellings at its front end, but the mouth cavity connects directly to the digestive tract. Earthworms feed on leaves and other plant matter.
The blood vessels. There are two main ones - one above and one below the length of the gut.
The upper (dorsal) vessel takes blood to the anterior end, and the lower (ventral) vessel
transports blood mostly to the posterior end. The dorsal vessel is contractile, pumping blood
forward. In each segment there are smaller vessels which form a ring joining the upper and
lower blood vessels. At the front of the worm several (typically five) of these ring vessels are
enlarged forming aortic arches which act as ‘hearts’ to pump the blood around the worm. To
see a video of an earthworm’s ‘hearts’ beating, use the following web address and wait for the
software to load on your computer.
http://yucky.discovery.com/noflash/worm/multi/heart.mov
The blood is distributed from the ventral vessels into capillaries in the body wall and organs,
and into a vascular sinus (blood-filled space) in the gut wall, where gases and nutrients are
exchanged.
Muscles. Just inside the outer skin are two tubes of muscles which the earthworm uses for
movement.
Other tubes are the kidneys and the male and female sex organs, all of which open at small
pores (holes) on the surface of the worm.
2
The diagram below is a cross section of a worm and shows the arrangements of most of the
body parts and the setae (bristles) which the worm uses in locomotion.
For a feast of earthworm illustrations, some animated, which give further information about
the internal structure of earthworms, use the following web site link.
http://sciencefun4all.net/Life_Sci/05BodyStructure/homework/EarthwormPictures.html
1.2 Feeding in earthworms.
Earthworms swallow soil, including small stones up to 1.25 mm across, as they burrow and
then extract nutrition from the pieces of decaying plant and animal matter that they take in
along with the soil particles. They can also tackle larger items of food such as small leaves,
stems and roots of plants.
The first part of the gut, the pharynx, acts like a suction pump and with it the earthworm can
‘hoover up’ soil and plant matter. The earthworm, Lumbricus terrestris, sometimes feeds at the
soil surface and it can grip larger items such as leaf stalks which may be pulled down into the
worm’s burrow to plug the entrance.
Next come the crop and gizzard. The crop is a storage region in which soil and food collects
before it is passed into the gizzard. This has strong muscular walls which grind together the
food, soil particles and ingested stones to break open the cells.
Digestion and absorption of food takes place in the rest of the gut. Waste soil particles, now
very finely ground up, are ejected from the rear of the worm, either on the soil surface as worm
casts [most likely by the long or blackhead worm, Aporrectodea (formerly Allolobophora) longa]
or, more often, inside the worm’s burrow.
1.3 Earthworm locomotion.
Anglers in the USA call earthworms ‘night crawlers’ because this is when they are generally
active. The worm uses its two layers of muscles and its setae (bristles) to ‘worm’ its way
through or over the soil. Typically, earthworms have four pairs of setae for each segment, but
some species have many more.
3
Muscular contractions alternately shorten and lengthen the body. The bristles grip onto
crevices between the soil particles and the body is pushed forward, anchored and the rear end
drawn up. Burrowing is aided by the secretion of slimy, lubricating mucus. Some worms
moving underground even make gurgling noises when they move fast through their burrows!
The earthworm can push objects aside that are ten times its own weight. If the worm cannot
squeeze along in compacted soil, it can always eat its way through! Movements up and down
burrows depend on the moisture in the soil and its temperature. When the soil is dry or very
cold, earthworms will try to move down considerable distances - between 1-2 metres - to
escape the adverse conditions.
For further information, see section 5.2.
1.4 How does an earthworm breathe?.
The earthworm is such a slow mover that it does not
have to release energy quickly and so does not need
special organs to take in a lot of the oxygen required
to release energy from food.
An earthworm can rely on obtaining all the oxygen
(O2) it needs by the gas passing through its permeable
skin surface. In the same way, it can remove the
carbon dioxide (CO2) that it produces as a waste.
Because the skin is permeable to gases, some moisture
will also escape from the worm and this is why
earthworms are usually only active in moist soils and
at night (particularly after rainfall) when they are in
no danger of drying up.
1.5 Does an earthworm have feelings?.
It certainly does, though it has no special sense organs that can be identified. Its whole skin is
sensitive to a greater or lesser extent, but particularly the first few segments. This is where the
enlarged ‘brain’ of the nerve cord is situated and this serves to collect most of the stimuli that
the worm receives. These include light, temperature, vibrations, chemicals and humidity.
The worm also has super fast-acting nerve fibres passing from the brain down the nerve cord.
In danger, the worm can contract its muscles very fast and move its body away very suddenly.
After a fall of rain, earthworms are often found on the soil surface. But what is the stimulus that
worms detect and prompts this behaviour? No one is certain and various suggestions have
been made.
•
Rainwater dissolves carbon dioxide in the soil, producing carbonic acid. The soil may
become too acidic for the worms, which attempt to escape by moving upwards.
•
Waterlogged soils contain very little oxygen, so perhaps worms come to the surface to
breathe. However, this doesn’t fit with the observation that worms can survive
underwater for several hours as long as there is oxygen in it.
•
Earthworms can travel more quickly and colonise new areas when travelling over
ground. They are less likely to dehydrate in damp conditions. So a rain storm is a good
time for a quick get away. (Earthworms die if exposed to strong UV light for extended
periods, so they usually move above ground at night - when there are also fewer
predators around.)
4
•
A few species, such as Lumbricus terrestris, move out of the soil to mate. But mating is not
always connected with rainfall, and many worms do not come to the soil surface, so this
seems an unlikely explanation.
•
Vibrations may bring worms to the soil surface. Perhaps the ‘noise’ of heavy rainfall acts
as a stimulus?
1.6 Regeneration.
Earthworms are renowned for their ability to regenerate lost parts. However, this ability to regenerate
segments varies very much between species and
with how much damage is caused. Some worms are
incapable of growing any new parts. Lumbricus
terrestris, the large earthworm most often exposed
when digging in soil, may replace anterior segments
but does not seem to be able to regenerate its tail.
Eisenia fetida, commonly found in compost heaps
and decaying leaf litter, can regenerate both head
and tail regions, even if these have been extensively
damaged.
If when digging up worms, an individual is accidentally damaged, it is then possible to investigate the regenerative powers of the unfortunate worm, but it is obviously not recommended
that worms should be cut up deliberately for such studies.
1.7 Reproduction.
Earthworms are neither male nor female but both; they
are hermaphrodites with an ability to produce sperm
and eggs in the same individual. Reproduction does,
however, involve two earthworms - with pairing and
copulation taking place generally below ground;
Lumbricus terrestris is an exception and may be
observed at night mating on the soil surface.
The two worms come to lie with the lower sides of their anterior ends in contact with each
other. The clitellum (saddle) produces a lot of slimy mucus which temporarily ‘glues’ each
worm to its partner. While clasped together, sperm is pumped out of the male sex organ
(testis), leaving through the male sperm opening and travelling along two sperm grooves
towards the saddle. The sperm then enters the sperm stores, two small pockets in the other
worm.
Path of sperm
along sperm groove Testis
Sperm
stores
Ovary
The worms then separate and later each produces more secretions from the saddle until a slime
tube is formed around the worm.
5
By muscular action, this cocoon tube is pushed
forward along the worm. As it passes over the
female openings, eggs pass into the cocoon from
the ovary. Sperm cells are pumped out of the
sperm stores and these fertilise the eggs. Eventually the tube is moved completely free of the
worm’s anterior end and the ends of the cocoon
close up, sealing inside the fertilised eggs where
they begin to develop.
Young worms hatch out of the lemon-shaped cocoon after a number of weeks. To watch this
happening, use the following web link.
http://yucky.discovery.com/noflash/worm/multi/wormbirth.mov
It seems that some worms are born with all their segments and simply become larger as they
grow. Their sexual structures develop later. In other species, however, the young worms have
few segments and, as they grow, new segments are produced in a zone just in front of the rear
end of the worm.
Earthworms grow at different rates, depending on the species, but most mature within six
months. Life spans again vary considerably, with worms surviving longer in protected cultures
than in their natural environment where they are exposed to various hazards.
In the ‘wild’, worms probably live for no more than up to one year but in protected conditions,
individuals of Aporrectodea (Allolobophora) longa have survived for over 10 years and Lumbricus
terrestris for 6 years.
1.8 Earthworms and the soil.
Charles Darwin is rightly famous for his theories of evolution but his studies of a wide range of
organisms, including barnacles, orchids, carnivorous plants (see CLEAPSS guide L226!) and
earthworms, are less well known. He spent many years researching the worms in the garden of
Down House in Kent where he lived and finally published a best seller called The Formation of
Vegetable Mould Through the Action of Earthworms.
Darwin observed the appearance of casts on his lawn (his resident worm must probably have
been Aporrectodea) and, over a period of thirty years, he watched a path of flat stones disappear
into the ground to be covered by a layer of soil about 8 cm deep. He collected and weighed
casts each day and from his calculations, Darwin concluded that during a ten-year period, the
worms in a meadow would bring enough soil to the surface to cover the entire field to a depth
of 6 cm.
We have earthworms to thank for the preservation of various Roman antiquities such as mosaic
pavements, coins and pots - protected from the action of frosts and rain under layers of soil.
Darwin also appreciated other important aspects of worm action. Earthworms are essential to
composting. The leaves that worms take down into the soil for food or to plug burrows
eventually rot if not eaten and add to the humus. Worm casts can contain 40% more humus
than the top 15 cm of the topsoil. A worm can produce as much as 4.5 kg of casts per year.
Earthworm burrows aerate and break up the soil giving good drainage and space for plant root
growth, and the worm casts produce excellent-quality top soil with finely-ground particles just right for seeds to germinate in. Worms also act as pistons, driving air through the soil as
they move through their burrows.
There can be significant differences in the numbers of worms in adjacent gardens. Worm
populations are affected by a host of environmental factors, which are influenced by good
management of the soil.
6
1.9 Worms as food and pollution fighters.
Earthworms form part of many food chains. They are preyed upon by several species of birds,
eg, starlings and thrushes. Mammals such as hedgehogs and moles eat many earthworms as
well. Earthworms are also eaten by many invertebrates including beetles, snails and slugs.
Earthworms also provide food for many internal parasites, including other worms, which are
found in many parts of earthworms’ bodies such as the blood, intestine or in the cocoons.
Earthworms are being used as efficient converters of organic wastes, such as pig slurry, turning
them into proteins that can be fed back to pigs and other farm animals, poultry and farmed
fish. The red and yellow-banded brandling or tiger worm, Eisenia fetida, is widely reared on
earthworm farms where it eats manure, reproducing quickly and producing large populations
of worms. These are then used to produce high-quality animal food. It is unlikely that humans
will gain a taste for such earthworm fodder; it is more likely that we will eat them indirectly after conversion into eggs, pork and chicken!
In feeding on such wastes, earthworms are also of great benefit in combating pollution. The
safe disposal of massive quantities of organic waste produced by the intensive rearing of
livestock has always been a problem but earthworm farms are providing a solution - with a
very valuable product raising income as well.
7
2. First catch your worm
There are various methods you might use to obtain earthworms for practical studies, and a number of factors you must
consider before starting.
Time of year: Earthworms become inactive when they become too cold, when they burrow
deep into the soil. You are more likely to find earthworms during the warmer parts of the year
from March to October.
Weather conditions: Earthworms burrow down into the soil when it becomes very dry.
Numbers of worms in the top layers of soil increase markedly after a period of rainfall.
Location: Some types of soils may not contain many worms because they are too acid, or they
don’t contain enough organic matter (humus). Some soils, particularly with a lot of clay,
become easily compacted and waterlogged; this will deter colonisation by many worms. Try to
choose an area where you can find worm casts on the soil surface, but the absence of casts does
not mean there are no worms below since many worms do not leave their casts above ground.
Finding a good site for worms involves a certain amount of trial and error - and scope for
investigative activities.
Small species of worms can be found by sorting through leaf litter. Larger worms, which are
better for investigations, will normally have to be dug or coaxed out of the soil. More-exotic
species may be found in well-cultivated gardens - especially those with greenhouses containing
imported plants.
2.1 Methods of extracting earthworms.
It is important to hunt for worms on open ground or close-cut grass because it can be hard to
see them emerge from the soil if the grass is long.
Hunting by night
Not entirely practical for most schools, but pupils might wish to go on earthworm forays at
home. They should choose a warm evening, preferably after rainfall. Some species of worm,
but not all, come to the surface to feed at night. They may also be drawn to the surface during
or after a shower of rain (see the earlier discussion in section 1.5).
A bright white light suddenly shining onto a
worm will often cause it to retreat underground.
Use a flashlight with a piece of red plastic,
cellophane or tissue paper wrapped over the bulb;
the worms are not sensitive to red-coloured light.
Approach your worm gently and quietly; too
much noise and vibration may send it
underground, back into its burrow (but see later
for beneficial vibrations).
Pitfall traps can sometimes be successful, if left out overnight, in catching worms that have
been active on the surface. Bury plastic pots up to the rim and shelter them from rain with
pieces of tile supported on stones.
8
Digging
This can be hard work and not everyone is happy to
have their lawns ploughed up! This method is
certainly effective in flower beds etc but you must be
prepared to dig at least 30 - 40 cm down, depending
on the weather. Worms may be damaged by spades;
garden forks are a little less harmful.
Watering
Worms are sometimes observed coming to
the soil surface after heavy rainfall, even
during the day.
You could try to simulate a rainstorm with
a couple of gallons of water on a small
patch of lawn. However, do not be
surprised if this isn’t too successful in
bringing worms to the surface!
Detergent
Liquid detergent has traditionally been used as a vermifuge - a chemical that forces worms to
the surface by chemical irritation. However, research has shown that it is significantly less
effective than other chemicals. Additionally, earthworms are often adversely affected by the
detergent and are likely to die after extraction. There is evidence that some worms die in the
soil before emerging. The use of detergent therefore cannot be recommended.
Mustard
It may sound strange but Colman’s English mustard has been
used successfully as a vermifuge! It takes a bit of effort to
prepare a mustard solution, especially from the powder,
because mustard is an emulsion and requires vigorous stirring.
Therefore use ready-mixed mustard: about a tablespoon (15 25 ml) stirred into a litre of water is the best concentration.
Usually between 4 and 10 litres of this mustard suspension are
applied to a square metre of soil or grassland, using a watering
can with a rose fitted on the spout. Increased concentrations
may extract more worms but the mustard suspension often
forms lumps before it can be applied. Mustard is non-toxic; nor
does it appear to harm grass or clover. This is the preferred
chemical method of extracting worms.
Other chemicals
Potassium permanganate and methanal solution (formalin) have often been used in the past as
vermifuges. They extract worms from the soil reasonably well though the solutions may harm
the worms in the process. Potassium permanganate kills plants, so it is not at all suitable for
well-kept lawns. Formalin is an unpleasant chemical to handle. Neither will be readily
available to primary schools. Since mustard is an effective vermifuge, there seems little point in
attempting to use these alternative chemicals.
Vibrations
Section 1.5 discussed rainfall causing worms to leave the soil. Perhaps it is the noise of the rain
that brings them up? Some birds, eg, the herring gull and lapwing, can be seen ‘stamping’ or
‘paddling’ the ground with their feet in order to bring worms to the surface. Providing the
worms are near the top of the soil, you can also try placing an old-fashioned bell alarm clock
face down onto the soil/lawn surface. There have been reports of success. Anglers dig a metal
bar or a garden fork into the soil and then hit it to produce vibrations.
9
You can also try using the feet of your pupils. There is no
need to stamp wildly on the ground. A technique that
works is for children to tap with their toes and heels,
using a slight forward and backward, heel and toe,
rocking movement. After about two or three minutes
worms may start appearing. Let them emerge completely
before attempting to pick them up.
There is some evidence that this technique works best
with Aporrectodea (Allolobophora) species. It is only likely
to be effective after a lot of rainfall and is good for lawns
where there are lots of worm casts.
2.2 Collecting extracted worms.
As soon as worms emerge from a vermifuge treatment, they should be washed in water to
remove the irritant chemical. They should then be transferred to a covered container (with air
holes) in which they will be kept moist and in the dark. Fill the container with damp, live,
sphagnum moss (obtainable from florists or garden centres as used for hanging baskets; do not
use sphagnum moss peat). The earthworms will burrow through this to the bottom and at the
same time be cleaned. At daily intervals, put the worms on top of the moist sphagnum moss
again. It is thought that this improves the resilience of the worms in later tests when they are
handled, and it helps to make them more pleasant to touch. For longer storage, or other
purposes, a wormery needs to be established; see section 3: Keeping Earthworms.
Sims and Gerard (see section 6) describe how to preserve dead worms for examination. They
advise relaxing the worms first with a 12-25% solution of vodka, gin or white rum. However,
killing earthworms for educational purposes, however inebriated, is not recommended.
2.3 Identifying earthworms.....
... is not easy! Earthworms belong to the group called the annelids or segmented worms. There
are 28 native British species, with around 10 that are common. Further species have been
introduced to the UK accidentally. The problem of identification arises sometimes that small
worms may just be immature specimens of much larger species. Generally, it is only the adults
which have a clitellum or saddle, though this is not always very distinct.
For most purposes, there is absolutely no need for schools to identify the type of earthworm
that is being investigated. However, for those interested in learning more about different
species of worm, there are two web sites which are useful; details are given below.
www.naturewatch.ca/english/wormwatch/about/guide/about_guide_redworms.html
www.nhm.ac.uk/nature-online/life/other-invertebrates/earthworm-slideshow/earthwormimages.html
It is sometimes useful to be able to tell some species apart, particularly Lumbricus terrestris and
Aporrectodea (Allolobophora) longa. It is, for example, much better to use L. terrestris in studies of
feeding. These two species are unfortunately fairly similar in appearance.
The best way of telling them apart is to look with a hand
lens at the front of the worms on their top (dorsal) side that is the side that feels smooth and has no bristles.
The diagrams show the different shape of the first
segment - the prostomium.
Lumbricus
terrestris
10
Aporrectodea
longa
3. Keeping earthworms
The standard method suggested in most books is to use a
specially-constructed (or purchased) wormery with two sheets
of glass or perspex between which the soil is sandwiched. This design is based on that used by
workers at the Rothamsted research station and is supposed to permit visual inspection of the
worms’ burrows through the glass. Unless, however, it is vital to demonstrate the action of
worms in mixing soil layers as they burrow, it is best not to use such a wormery.
As it is, however, the ‘accepted’ method, it will be discussed first.
3.1 Rothamsted wormery.
Many books give quite misleading information when suggesting wormeries of this type. It is not
worth purchasing a special wormery of this type, nor spending much effort in constructing
one. The wormery can be made to any size, depending on materials available but, if several
worms are to be housed, it should be approximately 25 cm x 25 cm.
A wormery is most easily constructed by using three pieces of wood laid on a sheet of glass (or
perspex) as shown below. Another sheet of glass is placed on top and the ‘sandwich’ is bound
together with adhesive fabric tape.
Ideally the gap between the glass sheets should be as small as possible to increase the chances
of seeing burrows, but too small a distance makes it difficult to add soil. The glass sheets
should, however, be no more than 25 mm apart. Many books show a width of more than
70 mm; this will ensure that worm inhabitants are rarely seen.
A good-quality soil can be made by mixing together John Innes and Levington composts with
some garden loam soil. The soil should preferably not be placed into the wormery in a dry,
powdery state and then watered. This will often destroy the soil structure and so provide a
quite unsuitable earthworm habitat. Soil should be moistened first so that when squeezed in
the hand, the soil ‘crumbs’ just hold together. The soil should be reasonably packed in the
wormery but not pressed down firmly.
Many people recommend alternating layers of different types of soil, sand, powdered chalk or
clay. (Note, do not use builders’ sand or powered blackboard chalk.) It is expected that the
worms will burrow through these layers and eventually mix them up. It is not, however, a
good idea to have thick layers of sand, chalk or clay; it is likely that the worms will be reluctant
to burrow through them. Only very thin ‘marker’ layers should be used, if at all.
11
Do not add too many worms. As a rough guide, large individuals need about 150 cm3 of soil
each and small ones, half this amount. Once set up, the sides of the wormery should be covered
with black paper or black polythene when the worms are not being observed. A little water
should be sprinkled on the soil surface daily to keep it moist but not wet. It is sensible to cover
the top of the wormery with polythene or ‘cling film’, pricked with a few air holes. This will
help to keep the humidity high and prevent worms escaping.
3.2 Alternative viewing wormeries.
Depending on the availability of various
sizes of transparent plastic boxes or
cylinders (eg, cut-down soft-drinks bottles
or margarine cartons), it is possible to
construct similar wormeries to the
Rothamsted design without the need to
saw up pieces of wood or cut glass.
All four sides should be covered with
black polythene or paper.
Another technique is to use a large, deep ice-cream carton
and cut ‘window’ holes in two sides. Two sheets of glass are
placed inside to form a ‘V’ (preferably by cutting slots in the
carton and sliding in the glass pieces). Soil can then be added
into the V, followed by the earthworms near the glass sheets.
With this design, it is hoped that the worms will burrow
straight down and then follow the glass with their burrows.
(This design is, incidentally, excellent for showing the
growth of roots from germinating seeds.)
The entire carton can be placed inside a bag to keep it dark.
Mini-wormeries using jam jars are also easy to set up and then covered to keep them dark.
With the increased width of soil in the containers, do not expect, however, to see so much
evidence of worms burrowing against the glass.
3.3 A worm box.
The easiest, and perhaps best, way to keep worms is to use any large box, bowl or bucket. Some
sort of cover should be provided to prevent escapes and to maintain dark, humid conditions
inside. Small ventilation holes will be needed.
As before, use moist garden soil, not compacting it as it is added; the soil should not come right
to the top of the container. Add a generous layer of rotting leaf litter. Alternatively, used coffee
grounds have been employed successfully as a medium in which to keep worms healthy. If the
worms are to be kept for a significant time, food will need to be added to the soil in the
wormery (see below).
For details of constructing a larger worm ‘bin’, particularly where worms could be used to
recycle vegetable food waste into compost, refer to the web site below.
http://yucky.discovery.com/noflash/worm/pg000223.html
Commercial earthworm composting kits (usually involving the tiger worm, Eisenia fetida) are
available from several suppliers, eg, Original Organics (www.originalorganics.co.uk), Recycle
Works (www.recycleworks.co.uk) and Wiggly Wigglers (www.wigglywigglers.co.uk). These
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kits consist of large containers and a supply of earthworms and are designed for large-scale
compost production from kitchen waste.
Temperature and feeding
All wormeries should ideally be kept out doors, and certainly not at a temperature higher than
12 °C. Temperatures around 18 °C will be fatal in the long term, although for a short period
worms will be more active, which can be beneficial for some observations. Temperatures
should not, however, fall much below 4 °C or the earthworms will become totally inactive.
The worms will feed on organic matter that is in the soil and on the soil surface to some extent.
It is therefore important to mix some well-rotted leaf litter into the soil. In addition a good
handful of oatmeal can be added to the soil surface and gently mixed into the top layer. This
should be replenished every six weeks or when it has all been consumed. Other waste vegetable
materials could also be added to the worm container and gently mixed into the soil.
Reproduction
In a suitable worm box, it is likely that the worms will breed. Cocoons are coloured brown,
orange or yellow (depending on the species) and about the size of a grain of rice. On average,
around five worms will hatch out in between a few days and a month (again dependent on
species).
It is very easy throw out the cocoons when cleaning out the box. The cocoons can only be
discovered by sifting through the soil very carefully by hand. If hatching is to be observed,
keep the cocoons separately in small covered dishes lined with damp tissues.
13
4. Investigating earthworms:
Outdoor activities
4.1 How many worms are there in the soil?.
Materials needed:
Equipment for extracting worms, eg, watering can (see section 2.1, First Catch
Your Worm for methods); quadrats (square frames) or large hoops; (alternatively, materials to peg out squares: skewers, string, scissors and measuring
tape). Containers to keep collected worms in the dark.
First of all, survey your area. Casts on the soil surface are a sure sign of worms below, but only
two or three species cast soil on the surface so there may be plenty of non-surface casting
worms. Look too for burrow openings and also for piles of small stones or leaves and twigs
sticking out of the ground. Only one or two species of worm plug the entrance to their
burrows.
You can compare various areas, eg, at different points on well-cut lawns, on rougher patches of
grass or well-trodden paths, in compacted or freshly-dug soil etc. For these studies you will
need to choose an extraction method suitable for all areas, as described in section 2.1, First Catch
Your Worm.
Then mark out equal-sized areas in each plot. You can use skewers pushed into the ground and
tie string round them to produce squares - which are called quadrats. Alternatively, use large
hoops. The size of each sampling area should be fairly large – around a square metre. Now
treat your quadrat areas to bring the worms up, using an appropriate technique, such as
watering with mustard or vibrating the soil surface. If necessary, repeat the treatment after
10 minutes. Count the total number of worms that emerge. Repeat the extraction method on
other quadrats in the area. Calculate average numbers of earthworms.
If the opportunity presents itself, repeat the sampling in the same areas but at different times of
the year and in different weather conditions (see the introduction to section 2).
4.2 Investigating casts.
Materials needed:
Oven or radiator; sensitive balance to weigh casts.
If you find worm casts, there are probably worms of the genus Aporrectodea (Allolobophora)
underground. There are a lot of questions about casts that you could pose to your pupils.
•
Where do they come from?
•
When are they made? - at night or during the day?
•
Are all casts the same shape and size?
•
How many casts are made each night/week?
•
How much soil is there in each cast? (Dry the casts in an oven and then weigh them. A
neighbouring secondary school science department should be able to help if you do not
possess a sufficiently-sensitive weighing machine.)
•
How much soil would the worms bring to the surface in a week/month/year? (You
could collect casts from a measured area of soil each day for a week, weigh these when
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dry and then multiply up to the amount of soil brought to the surface of a lawn each year
- assuming of course that the worms are equally active at all times - and make
comparisons with Darwin’s calculations.)
•
How is the soil in the casts different from that in the rest of the area? (It will be much
smoother, having been ground into fine particles when passing through the worm’s gut.)
•
How do the weather conditions affect production of casts?
4.3 Investigating plugged burrows.
Materials needed:
A selection of small leaves and stones; large earthenware plant pots.
If you find plugged burrows, it will be Lumbricus terrestris that
is the worm under ground. Early autumn is the best time for
these studies when there are lots of leaves on the ground.
You can easily keep records of earthworm activities by seeing
how materials are moved. Set up quadrats and make daily
maps of the area, as shown.
You can investigate whether the worms appear to have any
preferences in the materials they choose to plug their burrows.
Remove the items that are in the top of a burrow and provide
the worm with a selection of different sizes and shapes of
various types of leaves, plus conifer needles, small twigs and
tiny stones. These could easily be blown away so it is a good
idea to enclose these under a very large earthenware plant pot
placed over the burrow’s entrance. (Plastic pots are too light
and will need to be held down with a stone.)
4.4 Investigating feeding.
Materials needed:
Large earthenware plant pots; a selection of foods including leaves and
vegetable pieces; large-mesh nylon hair net; old pair of tights.
If there are plugged burrows around, then
worms are coming to the surface and it
will be possible to investigate any food
preferences that they have. [If not, do not
expect much (if any) success.] It is more
successful to study feeding in the natural
conditions out of doors if this can be
arranged.
Worms normally eat organic matter that is
encountered in the soil and on the soil
surface and then pulled down into the
burrows. Some worms seem to prefer plant
material that is well-rotted, others will feed
on more intact leaves and vegetable
matter.
There is a lot of confusing advice written in various books about the preferences of worms.
They are, for example, reported to enjoy chocolate and meat as well as only certain types of
leaf. There is clearly much scope for investigation here, but don’t believe what you read,
believe what you find out.
Try providing the worms with a variety of leaf material, both fresh and decaying, from
herbaceous plants as well as trees. Investigate various vegetables such as onion, cabbage,
15
lettuce, carrot, potato etc. Provide the worms with small pieces or slices. It would be best to
place these items underneath a plant pot over the worm’s burrow - which should be unblocked.
The biggest problem (and perhaps where some of the wild reports of worms’ feeding habits
arise), is in deciding whether food has been eaten by worms. There are plenty of other hungry
animals around. Look for signs of food being nibbled. If it completely disappears, it may have
been dragged down a burrow (or maybe removed by some other minibeast). Always set up a
control investigation. Enclose similar pieces of food under a pot but place this on the ground
well away from signs of worm activity. Then compare the two; if the food has only disappeared
under the worm’s pot, you can be more confident about the conclusions you draw about
worm’s feeding behaviour.
If in the area you study, no earthworms come to the surface to feed, it is possible to bury food
items in the soil. They should be placed inside a nylon hair net with a mesh of at least 7 mm so
that the worms can enter, and this then tied into a bag. As a control to see how much food
disappears just by the action of bacteria and moulds, similar amounts of each food should be
buried inside a bag made from a pair of old nylon tights. The small mesh prevents earthworms
reaching the food. The bags should be left in place for a few weeks before being examined.
This method is obviously less attractive as it involves digging into the soil and so disturbs the
worm’s natural habitat. It may, however, be the only method available of investigating feeding
outdoors.
4.5 Observing mating.
Materials needed:
Electric torch; red cellophane/tissue paper(or rear lamp from a bicycle); luck!
Again, an activity for a warm, damp evening, preferably
after rain. Use a torch covered with red cellophane/tissue
paper to track down the animals on the surface. Only
Lumbricus terrestris mates above ground in the open and so, if
there are no plugged burrows around, you’ll probably be in
for a long evening staring at the grass. Lumbricus will breed
throughout the year providing the conditions of
temperature, moisture and food supplies are suitable, but
most reproductive activity occurs in the summer months.
16
5. Investigating earthworms:
Indoor activities
Note: After handling earthworms in all of the activities described in this section, pupils
should always wash their hands thoroughly.
Also, for investigations with earthworm, the worms should not be allowed to dry out for
extended periods. The worms should also not be exposed to the light for too long. After a
while, they may stop moving and should then be returned to an empty, covered container of
damp sphagnum moss. Always have plenty of ‘spare’ worms available.
5.1 Explore your worm.
Materials needed:
Containers of damp worms; hand lenses, magnifiers, binocular stereomicroscope; sheet of glass (edges taped); bright lamp; clear glass or flexible PVC
tubing of a diameter to fit the worms, eg, 5 mm, 10 mm; cotton wool; Plasticine;
string; scissors; graph paper.
Earthworms must be kept moist - never let them dry out. If prolonged observations are made
and there is a danger of the worm becoming too dry, exchange the animal for a fresh one from
your worm stocks. Put the worm that has been studied into a covered pot containing damp
sphagnum moss; see section 2.2.
There is quite a lot to observe and experience even in something as relatively simple as an
earthworm. Depending on age and motivation of your children, encourage close and detailed
observation if you can. You will need to have some form of optical aids, either as hand lenses or
something more sophisticated. If you can afford one, a large binocular stereomicroscope is an
excellent investment.
Some children love to use long words and, for the various parts of the worm, there are plenty
of examples to keep them happy. There is no need, however, to use any technical terms if you
choose not to.
So what should your pupils be looking for and what questions might you ask? Here are a few
suggestions.
Do worms have a front and back, top and bottom?
The front end is pointed, the posterior is flattened. The
clitellum or saddle (only seen clearly in adults) is
always nearer the front end. The worm usually moves
forward ‘head’ first. There are four pairs of bristles
called setae or chaetae (singular seta; chaeta) on all
segments except the first two and the last one. They are
usually arranged on the lower surface and sides of the
worm. Pupils can run their fingers gently over the
upper and lower sides to feel for their presence or
absence.
17
Where are the mouth and anus?
You will need a magnifier to see them. The mouth is between the first and second segments
(prostomium and peristomium) on the lower side. See the diagrams below. The anus is on the
last segment.
Side view
Lower view
How is the worm’s body constructed?
Pupils should see the separate rings or segments and the saddle.
Is the saddle always in the same place? It should be in the same
species of worms - so if it isn’t, you are comparing different types of
worm. The position of the saddle is sometimes used to identify
worms. For the committed worm lover, try counting the number of
segments in front of the saddle and compare this in different worms.
Depending on how closely pupils look, they may also see a number
of holes or pores on the lower surface. Some are very small (from
the ‘kidneys’ in each segment) but others are more visible.
Look for the female and male openings; the female pores are always
in the segment in front of the male pores. Through these openings,
the eggs and sperm cells pass from the ovary and testis out of the
body into the cocoon that is formed by the saddle (see section 1.7).
You might also see a groove between the male pores and the saddle along which the sperm cells swim during mating - but it isn’t always
very clear. If you are very sharp-eyed, you may also see small openings in segments before the female pores; these are the entrance to
the spermathecae (sperm stores) in which the sperms are kept after
mating.
How does the worm’s body feel?
It will feel moist and maybe slimy if the worms have not been kept in sphagnum moss. It may
also feel ‘scratchy’ if the fingers are run over the bristles on the lower side of the worm.
Why is the worm’s body surface moist? This is partly for lubrication as the worm burrows
through the soil. Also, because the worm breathes through its skin, the skin is therefore
permeable to oxygen and carbon dioxide. As a result, water can also pass out of the body
through the skin, so the worm is always slightly damp.
Some pupils may be reluctant to pick up worms. If this becomes a problem, you can try
persuading a worm to crawl into a ‘surrogate burrow’. Use glass or plastic tubing. It is
important to wet the inside of the tubes first. When the worm is safely inside, plug the ends
with damp cotton wool. The worm can now be observed through the walls of the tubing. Some
loss of clarity is, however, inevitable.
You may be able to ‘borrow’ supplies of tubes or tubing from a local secondary school science
department.
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What’s inside an earthworm?
Dissection is not recommended. You may, however, be able to see something of the worm’s
insides if you put an animal on a wet glass plate and shine a strong lamp through the glass and
worm. In this way, the gut passing from the mouth to anus may be visible and the movement
of food along may also be seen. There are also the main blood vessels above and below the gut
and the earthworm’s ‘hearts’ - five pairs of blood vessels around the gut towards the front of
the body - which may be seen beating. (See section 1.1 for details of a video clip showing the
beating hearts.)
How long can a worm stretch?
Set your pupils the problem of measuring their worm. Let the pupils try to devise a method for
themselves. As it moves, they could, for example, stretch a piece of string alongside when it is
fully extended and cut the string to this length. Then repeat this when the worm is at its
shortest length and compare the two pieces of string.
Make an earthworm model
After the pupils have carefully studied their worms, ask them to make a model of the worm out
of Plasticine. The age, interests and observational skills of the children will influence the
accuracy of their models.
5.2 Earthworm movement.
Materials needed:
Pots of moist worms in sphagnum moss, kept in the dark; empty pots of sphagnum moss; sheets of glass or glazed tile; various surfaces of different textures,
eg, blotting paper, brown paper, smooth writing paper, sand paper, various
fabrics etc; sheets of stiff card or hardboard; stop watch or wrist watch with
timing mode; graph paper; large sheets of sugar paper; ruler; string; scissors;
candle; matches; 3 in 1 oil.
How does a worm move?
Pupils should be asked to observe the different parts of the
worm as it moves forward and think how the worm is using
its bristles, which can be extended or retracted. How is the
observed movement related to borrowing through the soil?
The worm moves by retracting the bristles at the front of the
body and extending forward. The bristles at the front are
then pushed out to grip the surface and the area behind
releases its bristles and is contracted and pulled up. This is
repeated further back down the earthworm’s body.
These movements are achieved because the earthworm has
two sets of muscles in its body.
One set runs along the length of the worm from head
to tail. When these longitudinal muscles contract,
they shorten the worm.
The other muscles run in a circle around the worm.
When these circular muscles contract they squeeze
the body so that it stretches forward.
19
How do earthworms move on different surfaces?
Place them on a wet sheet of glass or tile. How do they move? Put them on a moistened sheet of
brown paper. How is the movement different? Listen closely to the worm on the brown paper.
What can be heard?
These observations should show the importance of a relatively rough surface for the bristles to
grip on. The movement of the bristles against the paper should be audible.
Now try the worm on various other surfaces. Give them a choice by placing them across two
different surfaces and noting how and where they move. Try the same surface but with damp
and dry regions as the choice.
Once any preferences have been established, use a surface which can be made damp for any
further observations of movement.
How fast can an earthworm move?
This is a relatively easy problem if the worm moves in a straight line. Tests can be carried out
on large sheets of moist graph paper or pupils can follow the worm’s path with a pencil and
then use string to find out the distance moved in the time. Encourage pupils to think whether
one recording is enough or if an average of several would be better.
How is movement affected by conditions?
Speed of movement is influenced a great deal by the conditions in which the worms are kept.
i)
For a short time only, keep some worms in a colder place and others at a warm room
temperature. Then compare their speeds. Worms are poikilothermic or ‘cold-blooded’ and
their metabolism is controlled by the outside temperature. Humans can be active all the
time because our temperature is kept high and constant.
ii) Compare how their activity is affected when worms are kept in moist conditions and in dry
conditions for a relatively short time before a test.
iii) Worms do not normally move about in the light. There is a suggestion that they are killed
by over-exposure to the ultra-violet radiation in sunlight. Could tests in the light be giving
us quite erroneous information about earthworm movements? Ask pupils to design an
investigation to find out whether earthworms move more or less in the dark than in the
light. The problem of course is that they can’t be observed in the dark. Consider intervening
with suggestions if your pupils get stuck. Here are a few ideas.
•
Worms appear to be insensitive to red light. Pupils could use red light as representing
darkness for the worm.
•
Worms can be kept in a dark box with the bottom lined with smoked paper. An adult
can smoke paper by holding it over a candle flame into which bicycle oil has been
dropped to make a lot of sooty smoke. Be careful! The extent of the worm’s movements
can be assessed by tracks made on the paper. The paper should not be too smooth, or
the worm may not move normally.
The same test should be carried out with the box opened to the light to see if the worm’s
movements are affected by the smoked paper. Remember also to keep everything else
the same - the worms must come from the same conditions beforehand (damp and
dark). Ideally the same worm should be tested in the dark and the light, but worms do
stop moving if they are handled too much, become too dry or are left in the light for too
long. You may therefore need to test similar worms from the same batch.
•
An alternative arrangement to that described above might be to dust a thin layer of
unperfumed talc in a dark box.
20
Can worms ‘jump’ across a gap?
Worms can move across unfavourable surfaces to find suitable conditions. For example, a
species of worm in which individuals are very long (Microchaetus rappi) was found stretched
across a 6 metre-wide main road in Africa.
Damp. rough
paper
Set up the materials as shown, starting with a
small gap, and repeat the tests, increasing the
distance between the books.
An extension of this is to put the gap over
something unfavourable - in this case a dish
of water. How does this affect the worm’s
movement?
Making a worm ‘movie’
Video can be used to record worm movements. Lines or other markers can be used to give a
sense of progress and scale. After their work on watching worms move, pupils might like to try
making a ‘flick book’ to show a worm crawling across a surface.
5.3 Burrowing.
Only a few species of earthworm form distinct burrows which they then line with egested soil
and lubricate the walls with their secretions. Lumbricus terrestris, usually found when digging
in garden beds, often produces vertical burrows but it tends not to do this if there is plenty of
food on the soil surface. Aporrectodea (Allolobophora) longa, the long worm, and Aporrectodea
caliginosa (nocturna) also produce permanent burrows. It is only these three worms that cast the
soil from their guts onto the soil surface or sides of their burrows. Lumbricus terrestris is the
least likely of the three species to produce surface casts.
Materials needed:
Wormery; a range of soil types, eg, moss peat, Levington’s compost, John Innes
compost, garden soil, sand, clay; sawdust; similar-sized jam jars; biscuit tin or
wooden box about 15 cm deep; card; containers of gelatine; lengths of glass or
PVC tubing of various diameters; a supply of moist worms kept in the dark.
Burrowing in a wormery
Setting up suitable wormeries is described in section 3, Keeping
Earthworms. They can sometimes be effective in showing worms’
burrows and the mixing of layers of soil, but all too often results
are disappointing. A lot depends on the type of worms that are
placed in the wormery - try to use Lumbricus or Aporrectodea.
The wormery should be kept cool, but not too cool, and not
allowed to dry out or become waterlogged. The sides should
always be covered with black paper or black polythene when the
wormeries are not being observed. If you are lucky, you will see
signs of burrows against the glass or perspex and some mixing of
the layers. Be warned, however, that this ‘mixing’ is sometimes no
more than soil subsidence or the result of water trickling through
and moving the soil particles. Set up a control wormery without
worms and treat it in the same way to see if the mixing is not
related to worm activity.
21
It is useful to have a control wormery for another reason. After the worms have been
burrowing inside your experimental wormery for some time, you can show how their activities
have affected the soil structure. Pour the same volume of water on top of each wormery and
note how long it takes to sink in. Look also for air bubbling out of the soil. You might expect to
see that the water is soaked up more quickly in the inhabited wormery. This is because there
are plenty of air-filled burrows for the water to flow into. In the soil, the earthworms’ activities
help water to penetrate easily and, equally important, bring air to the roots of plants.
Burrowing preferences
You can easily investigate any preferences that earthworms might have regarding the nature of
the medium they will burrow into. You can use individual large jam jars or give the worms a
choice by dividing up a large container such as a deep biscuit tin with card partitions into four
separate chambers. Fill the jam jars and the four areas in the tin with a range of suitable
materials, eg, moss peat; Levington’s compost; John Innes compost; ordinary garden soil; sand;
clay; sawdust etc. Then pull out the card dividers.
There are other factors that you could also vary. You could make the soil moist and/or very
wet and compare this with dry soils (but make the soil moist before placing it in containers).
You could investigate the importance of the structure of the soil by comparing samples in
which the soil is very loose or very compacted. Do you also obtain the same results when the
containers are placed in the dark rather than out in the classroom?
Place three worms in each jam jar and 12 worms in the middle of the soil ‘choice chamber’.
Time how long it takes before the worms start to burrow or have all disappeared. Note any
preferences shown. From your observations, decide what are the important factors that
stimulate an earthworm to start burrowing.
Watching worms burrow
The problem with the suggestions already made is that you usually cannot see the worms’
progress through the soil as they burrow. You can, however, watch a worm’s movements by
persuading it to burrow down into a glass or pvc tube of the appropriate diameter which has
been made wet on the inside. Alternatively, you can make up gelatine, pour the solution into
jam jars and let it cool to set. The jelly formed should be just rigid, but not sloppy. You may
need one or two trials to obtain a jelly of the correct consistency. It might also help to break up
the surface of the jelly with a pencil to make it easier for the worms to penetrate it initially.
5.4 What do worms eat?.
Materials needed:
Worm culture; a variety of food materials suggested by the pupils.
Earthworm feeding habits are best investigated in natural conditions, but this is not always
possible, or the area that can be studied does not have worms that come to the surface to feed.
Feeding investigations in the conditions of the classroom may be rather artificial, so results
obtained in this way should be treated with some scepticism. If at all possible, use Lumbricus
terrestris as the species of worm that you study because this is more likely to emerge from the
soil to feed on presented food items.
It is not worth setting up a wormery with parallel glass sides just for feeding investigations.
The simplest arrangement is a large container with a cover, eg, a washing-up bowl. Food can be
left on the soil surface. If the worms do not emerge to feed, the food can be buried in the soil,
although this clearly makes things more difficult in assessing whether it has been eaten.
An alternative is to clear away the soil from one side of the worm box and use this as the
feeding area. In any feeding studies, a control without worms should be set up, particularly if
food is buried in the soil and comparisons made.
22
Do not complicate matters by providing the worms with too many types of food at once. A
clearer picture of food preferences will emerge if only two or three foods are presented
together. Ideally, food should be cut into small pieces or slices and be of the same size. Worms
that have not been presented with foods for a week before feeding studies are due to start will
be more likely to feed.
5.5 What can worms sense?.
Like all living organisms, earthworms must be able to sense their surroundings and respond
appropriately - by moving away from harmful stimuli and moving towards those which are
associated with shelter or food. It is not easy to discover what an earthworm is sensitive to,
often because the worm is responding to more than one stimulus, or because it does not behave
normally in the artificial conditions in which it is tested in the classroom.
You cannot tell if an earthworm has sensed something until it reacts in some way. A worm
might normally be able to detect a stimulus but if, for example, it is incapable of responding,
we would not be correct in saying that the worm is insensitive to the stimulus being tested.
With earthworms we can only gain some idea of the important senses if the animal moves away from, or towards, the stimulus.
In tests of sensitivity, it is usual to provide the animal being studied with a choice between two
conditions, but only two. To make the tests fair, it is essential to stop any other factors
interfering. If, for example, light conditions change while testing for a response to moisture, we
will not know to which stimulus the worm responded. Also when trying to interpret an earthworm’s movements, it is important to consider how this response would help the worm if the
animal was in its natural surroundings.
Investigations of earthworms burrowing, moving on different surfaces and feeding on various
foods are all testing reactions to the stimuli of touch/contact and/or moisture. How did the
earthworms sense their surroundings - by touch, smell or taste?
Remember:
Earthworms exposed constantly to the light or to constant vibrations, or if allowed to become
too dry, will stop moving. Worms should always be kept in moist and dark conditions before
they are needed and not over-used in experiments. For each new test, select a fresh earthworm.
Materials needed:
Blotting paper; sheets of hardboard or thick card; large shallow rectangular
trough or tray; lamp; dishes of hot and iced water; small aquarium with
hardboard divider; garden soil; seed-tray warmer or infra-red heat pad; light
source with narrow beam; red and other colours of plastic, cellophane or tissue
paper (or rear lamp from bicycle); glass or pvc tubing; pipe cleaners; toothpicks;
tuning forks; alarm clock and or buzzer; cotton buds; diluted vinegar; liquidised
plant and vegetable matter; worms kept moistened and in the dark.
Up or down?
Earthworms burrow down into the soil, but is this because they can sense gravity or because
they are trying to escape from unfavourable conditions such as light or heat? If they do respond
positively to gravity, what are the factors that over-ride this when the worms come to the soil
surface?
Test an earthworm’s response to a gradient. Place it on damp blotting paper on a board or thick
card. Hold the card at an angle; how does the worm respond? Keep increasing the slope. Does
the worm’s response change? Are all the other factors constant in your test or could the worm
be reacting to something else? Does the worm respond in the same way if it is on a sloping dry
surface?
23
Choice of conditions
Find out how worms respond to choices between light and dark, damp and dry, hot and cold.
Use a shallow rectangular trough or tray.
i) Line the bottom of the dish with two
equal-sized pieces of damp blotting
paper. Put several worms in the middle
and cover one half of the tray with a
card or board. Place a lamp over the
other half so that it is not shining under
the card into the ‘dark’ side. After five
minutes, where are the worms?
Could the worms be responding to
something other than light? Why were
several worms used and not one? Why
should the test be repeated?
When the test is repeated, it is a good idea to reverse the position of the lamp so that it is
over the other end of the dish. How would this help?
ii) Repeat the choice test but this time remove one
piece of the damp blotting paper, dry the dish on
that side and replace with a fresh, dry piece. The
worm must not respond to light, so cover the dish
with a piece of card. After five minutes look to see
where the worms have moved from the middle of
the dish. Repeat the test again.
If you wanted to find out if the worms’ movements
were caused by a stimulus other than moisture,
what test should you do?
iii To test for a response to temperature, repeat the investigation with the whole of the trough
lined with damp blotting paper. This time, however, stand the trough on top of two dishes.
One is filled with hot water, the other with iced water, and they are positioned so that each
end of the trough placed on top is at a different temperature.
Cover the dish as before, with the worms in the middle, and see how they have moved
after a few minutes.
A criticism of these tests is that the worms are not in their normal environment and so may
behave in an unusual way. A more natural method of testing the worms’ response to moisture
is to set up a small aquarium, as shown overleaf.
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If no aquarium is available, you can use any large container; it isn’t necessary to see through
the glass or plastic. A layer of about 3 cm of damp soil is placed across the bottom. The tank is
then divided into two with a piece of hardboard, but this is not pushed right to the bottom. Fill
up each side with damp soil. Add a dozen worms to the tank with equal numbers on each side
of the partition. They can move from one side to the other beneath the central partition.
Over the next 2-3 weeks, sprinkle water daily on one side only, and leave the other side to dry
out. After this time, carefully dig up the soil and count the numbers of worms in each side.
Does their distribution reflect the difference in moisture?
The same arrangement can be used to investigate response to a temperature gradient. One side
of the aquarium can be stood on top of a seed-tray warmer (or use an infra-red heat pad) so
that the temperature on that side will be higher (but make sure that the extra heat does not dry
out the soil on that side).
Light and dark
Typically earthworms react negatively to light, though there is some suggestion that they move
towards a very dim light. One particular response that worms make may have been seen if they
have been ‘hunted’ at night coming out of their burrows. Often the worms leave their rear end
anchored in the burrow and, if a light is shone on them, they will quickly contract and
withdraw into the soil.
You can investigate this withdrawal response providing you can make the classroom dark or
the lighting dim. A narrow beam of light is best, so tape a piece of black card or plastic with a
small hole punched in the middle over the lamp. Torches are very convenient but their light
output can be a bit feeble.
You can test a worm on a piece of damp blotting paper
or encourage it to burrow into a length of glass or pvc
tubing that is wet inside. Shine the narrow beam of
light on the front end of the worm. The withdrawal
response is most dramatic if the light is shone just
when the worm is preparing to move, making small
exploratory movements with its front end.
Now test the other parts of the worm - its middle and
rear end. Do they respond in the same way? Where are
the worm’s light-sensitive areas? NOTE: Do not over
expose your worm to light or it will stop responding.
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Reference has been made to worms’ insensitivity to red light. Repeat the tests above with red
cellophane etc. over the light (or use a rear lamp from a bicycle). If the worm fails to respond, is
it because of the colour of the light or because the intensity of the light has been reduced? How
would you find out? Is the worm unresponsive to any other colours of light?
Sense of touch
Earlier work investigating burrowing and the foot-rocking method of extracting worms from
the soil indicate that earthworms can sense contact and vibrations. But which parts of a worm
are sensitive and does the strength of vibrations have a effect? (Tap the ground and worms
come up; stamp on it and they withdraw!)
Try gently touching a worm placed on damp blotting paper with a moistened pipe-cleaner or
toothpick. How does it react? Touch various parts of the worm; are they equally sensitive?
How do worms respond when they contact objects, or a pool of water, in their path?
What are the effects of various levels of vibration?
Tap gently on the surface on which the worm is
placed and note any response. Increase the force of
your tapping. Try the vibration of tuning forks, a
bell alarm clock or a buzzer. If you continue the
vibration for long, you will find that the
earthworms become used to it and stop responding.
Darwin had some pots of worms on his piano and
observed that, when certain keys were struck, the
worms reacted violently. There is scope here for
tests on the frequency of vibrations and their effects.
Taste and smell
If earthworms select only certain types of food, they presumably have some sense of taste or
smell. It has been suggested that worms come to the surface after rain because their burrows
have become waterlogged. The build up of carbon dioxide from animals in the soil makes the
water acidic and it could be this that the worms respond to.
The earthworms’ sense of smell and taste are not, however, easy to investigate. You can soak
pipe cleaners or cotton buds in various substances such as soda water (carbon dioxide
solution), dilute vinegar, salt and sugar water, or solutions made by liquidising various leaf
and vegetable material. Bring these close to the front of the worm and look for a response which will, however, not be simple to interpret.
Perhaps the best method of investigation is to use the technique described in the earlier section,
Choice of conditions. Moisten with water one piece of blotting paper in the dish and the other
with the solution to be tested. Place several worms in the middle and cover the dish.
Movements to one side or the other will indicate preferences, which are likely to be caused by
the substances in solution since both sides are damp.
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6. Learning materials & resources
Down House
Down House, The Home of Charles Darwin, is available for free, pre-booked educational visits
from March - December (closed Mondays).
Darwin studied earthworms in great detail, including experimenting with their sensitivity to
light, heat and sound, their sense of smell and their eating habits. Darwin’s ‘wormstone’ can
still be seen on the lawn, part of his experiments in monitoring the activity of earthworms in
the soil. An exhibition and interactive exhibits on the first floor of the house explain more about
Darwin’s experiments; the ground floor of the house is laid out as it was when Darwin lived
there, including his study and the billiard room (where many of his earthworm experiments
took place).
For more information, visit www.english-heritage.org.uk/downhouse; for educational
bookings contact Rachel Jones, Education Assistant, on 020 7499 5676 or [email protected].
A free Teacher’s Booklet is available to download from:
www.english-heritage.org.uk/upload/pdf/down_house_20050222124748.pdf
Scientific guides
Sims, R W and Gerard, B M; Earthworms (1999). Published by the Field Studies Council for the
Linnean Society of London and the Estuarine and Coastal Sciences Association in the British
Fauna series, No. 31. ISBN 1851532625.
This is a complete, authoritative, technical guide to the biology, identification and classification
of earthworms, including diagrams of body structure.
Selected earthworm web sites
If you are reading a paper version of this guide, rather than typing into your browser the web
addresses below, you may wish to use the electronic version of L257 on the CLEAPSS
members-only web site or located elsewhere. Clicking on the web addresses on the pages of the
pdf file should launch your browser, assuming that your computer is connected to the internet.
UK sites
http://213.121.208.4/pdfs/publications/education/wgf/Postcard-worm.pdf
An earthworm postcard to download.
http://darwin.baruch.cuny.edu/biography/down/
Gwen Raverat, grand-daughter of Charles Darwin, describes her memories of Down House
and the earthworm stone.
www.darwinproject.ac.uk/darwin/search/advanced?query=subject:%22%3Cfauna%3E+earthworm%22
The Darwin Correspondence Project records his letters, including those on earthworms. Only a
handful of the 17 ‘earthworm’ letters listed were available online in October 2007, but the
project is still developing.
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www.nhm.ac.uk/nature-online/life/other-invertebrates/earthworm-slideshow/earthworm-images.html
Facts and figures from the London Natural History Museum and the National Earthworm
Survey for identifying common UK earthworms.
http://sciencefun4all.net/Life_Sci/05BodyStructure/homework/EarthwormPictures.html
A collection of illustrations, some animated, showing earthworm anatomy and biology.
www.uclan.ac.uk/facs/science/ewff/ahtml_introduction.htm
A guide to earthworms and their identification.
www.wildlifetrust.org.uk/cheshire/watch_earthworms.htm
An activity sheet about earthworms produced by the Cheshire Wildlife Trust.
www.wormsdirectuk.co.uk
Buying worms for composting.
North American sites, some with US spelling
http://garden.blogware.com/blog/_archives/2006/11/6/2478521.html
The Acorns site explains Charles Darwin’s fascination with earthworms. Many of his simple
experiments - such as seeing how worms drew paper triangles into the soil - could be
reproduced.
http://yucky.discovery.com/noflash/worm/index.html
This page is an index to the resources on the site. In All About Earthworms, Wendell the Worm
reports on earthworms. Videos display a worm’s beating heart and a live birth.
http://yucky.discovery.com/teachercenter/pg000066.htm
Teachers’ notes for teaching with worms.
www.ars.usda.gov/is/kids/soil/story2/goodworm.htm
Careful: Earthworms Underfoot! A story about a soil scientist and the work he does to learn
more about earthworms. Upper primary.
www.backyardnature.net/earthwrm.htm
A general site on earthworms.
www.frogwatch.ca/english/wormwatch/index.html
Worm Watch: a Canadian site, including teacher’s lesson plans and pupil investigations.
www.ibiblio.org/pub/academic/agriculture/sustainable_agriculture/faqs/earthworm-faq.html
Canadian English language site. Frequently Asked Questions about earthworms. If you have
technical questions related to earthworms, you can submit those questions here to the
Earthworm Bulletin Board Service, which is monitored by more than 40 earthworm specialists
from around the world.
www.learner.org/jnorth/search/Worm.html
FAQs about earthworms, geared to primary and middle school level. Includes teachers’ notes.
Lots of ‘how big’ and ‘how many’ questions answered.
www.nysite.com/nature/fauna/earthworm.htm
General information about earthworms in New York.
www.uga.edu/srel/kidsdoscience/kidsdoscience-behavior.htm
Complete teaching resource with PowerPoint presentations, activities, diagrams, lesson plan.
www.urbanext.uiuc.edu/worms/
The autobiography of Squirmin’ Herman the worm. A teachers’ note-linked site full of
information about earthworms from the perspective of a worm.
www.wormdigest.org
Everything you could possibly want to know about earthworms! See the Mighty Worm link for
some amazing facts. (Example: worms eat the bacteria that eat cotton cloth, leaving behind just
the buttons and the zip from a pair of jeans.)
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Selected books on earthworms
Plenty of books on earthworms have appeared over the years. Even though many of
these may no longer be in print, they can often be tracked down at local libraries or
purchased on Amazon and other internet sites.
Ahlberg, Janet & Allan (1979-2000), The Worm Book and The Little Worm Book.
A quite delightful, humorous book for children; alas not currently in print. Several editions
have been produced, from publishers including Collins, Picture Lions and Puffin Books.
Apellhof, Mary (1982), Worms Eat My Garbage, Flower Press, ISBN 0942256034.
How worms can be used in composting.
Cronin, Doreen (2003), Diary of a Worm, Harper Collins, ISBN 006000150X.
A humorous fictional picture book story for young children.
Dell’Oro, Suzanne Paul (2000), Tunnelling Earthworms, Lerner Publications, ISBN 0822537621.
In the Pull Ahead Books series, with high-quality photographs, this is an easy to read, non-fiction
book for younger children.
Dixon, Norma (2005), Lowdown on Earthworms, Fitzhenry & Whiteside, ISBN 1550051199.
A well-illustrated information book for young readers.
Glaser, L. (1992), Wonderful worms, First Avenue Editions, ISBN 1562947303.
A picture book describing the features, behaviour and life cycle of the common earthworm.
Heinrich, Ann (2004), Worms, Compass Point Books, ISBN 0756505895.
Full-colour photography information book.
Hess, Lilo (1979), The Amazing Earthworm, Charles Scribner, ISBN 068416079X.
Describes the characteristics and habits of the earthworm.
Hipp, Andrew (2005), The Life Cycle of an Earthworm, Rosen Classroom Books, ISBN 1404255842.
A simple, illustrated non-fiction book for young children.
Hoffman, Jane (1994) Exploring Earthworms With Me, Backyard Scientist, ISBN 0961866357.
Science investigations with earthworms, simple and fun to do.
Holmes, K.evin (2006), Earthworms, Capstone Press, ISBN 073688064X.
An introduction to earthworms’ physical characteristics, behaviour and relationships to
humans.
Hovanec, Erin (2000), I Wonder What It’s Like to be an Earthworm?, Rosen Publishing Group, ISBN
0823954544.
A light-hearted, illustrated book for young children.
Jennings, Terry (1988), Earthworms, Oxford University Press, ISBN 0199182566.
A lively and colourful introduction to science and worms for 5-8 year olds.
Llewellyn, Claire & Watts, Barrie (2002), Earthworms, Scholastic Library Publishing, ISBN 0531146510.
An illustrated information book.
Mclaughin, Molly (1990), Earthworms, Dirt and Rotten Leaves, HarperCollins Childrens Books, ISBN
0380710749.
Examines the earthworm and its environment, suggesting experiments to introduce basic
ecological concepts as demonstrated by the earthworm’s survival in its habitat.
Nancarrow, Loren & Hogan Taylor, Janet (1998), The Worm Book: The Complete Guide to Worms in
Your Garden, Ten Speed Press, ISBN 0898159946.
An extensive guide to using earthworms for composting.
Pascoe, Elaine (1997), Nature Close Up: Earthworms, Blackbirch Press, ISBN 1567111777.
Describes the digging habits, physical characteristics, reproductive process and habitat of the
earthworm and provides instructions for related hands-on science projects.
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Pfeffer, Wendy (2004), Wiggling Worms at Work, Harper Collins, ISBN 006028448X.
A general information book for younger readers.
Ross, Michael (1996), Wormology, Carolrhoda Books, ISBN 0876149379.
Combines background information and suggestions for investigations.
Ziefert, Harriet (1988), Mr Rose’s Class: Worm Day, Collins, ISBN 0001900072.
The story of a science teacher and his practical hands-on work with earthworms.
Poster
W1-T202 Classifying Minibeasts. PCET Wallcharts, www.pcet.co.uk/primary/
One earthworm illustrated alongside other common ‘minibeasts’.
Film: VHS/DVD
The Earthworm: Darwin’s Plough, VHS: 009-01162V; DVD: 009-01162R
Viewtech Educational Media, www.viewtech.co.uk/detail.html?pgcode=01162
Darwin's experiments are recreated to illustrate the natural cycle in which earthworms return
nutrients to the soil for plants to use; suitable for older children.
Journals
For details of the methods of extracting earthworms from soil which are discussed in this guide
and investigations with worms in secondary schools, the following references may be of
interest. (The Journal of Biological Education is published by the Institute of Biology; School
Science Review is published by the Association for Science Education.)
East, Duncan & Knight, David (1998), Sampling soil earthworm populations using household detergent
and mustard, Journal of Biological Education No. 32 (3), 1998, pages 201-205.
Gunn, Alan (1991), Estimating earthworm populations, School Science Review No. 72 (261), June 1991,
pages 86-89.
Hill-Cottingham, M P (1981), Collecting earthworms, Journal of Biological Education No. 15 (3), 1981,
pages 185-186.
Piearce, Trevor, Robinson, Clare & Ineson, Philip (1988), Earthworms and soil pH, School Science
Review No. 70 (250), September 1988, pages 63-66.
Piearce, T G, Roggero, N & Tipping, R (1994), Earthworms and seeds, Journal of Biological Education
No. 28 (3), 1994, pages 195-202,
A New Scientist article on earthworms may also be of interest. The web address links to the
article itself.
Knight, David (1989), Nice work for a worm, New Scientist, 8 July, 1989.
www.newscientist.com/article/mg12316724.800-nice-work-for-a-worm-never-underestimatethe-earthworm-farmers-want-more-and-sewage-works-could-use-them--worms-could-evenwriggletheir-way-onto-the-stock-exchange-soon-.html
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