Entomology
BIO 3323
INSECT RELATIVES PANARTHROPODA
The Panarthropoda includes today's arthropods and their close
relatives the onychophorans and tardigrades. Although they may not
look like they are related to each other they share the characteristics
of the moulted alpha-chitin cuticle, loss of external cilia, appendages
with terminal claws, and the dorsal ostiate heart. Animals in the three
phyla are all that remain of the panarthropods that first appeared
with the Cambrian explosion; a period when the arthropod body
plans was the most diverse. As new fossils from this period, and those
like the Burgess Shale fossils are being re-examined, the diversity of
types is increasing and transitional forms that link today's Crustacea,
Chelicerata, and Uniramia are being identified proving that the group
is monophyletic in its origins.
ONYCHOPHORA
Onychophorans are commonly called velvet worms, something that's
a little hard to imagine as you look at a preserved specimen. But
when it's alive all those little bumps, tubercles or papillae, on the
surface of the animal give it a velvety look, and hence their name.
Although originally though to be related to worms it now seems clear
that they are members of the panarthropods. Onychophorans live in
moist soil and leaf litter and are often found hiding in cracks and
crevices. Although their cuticle is composed of the usual alpha-chitin,
it's thin and moulted in patches rather than all at once. This is related
to the worm-like movements of the animals and the role of
underlying sheets of circular and longitudinal muscles and hemocoel
is a hydrostatic skeleton. The thin cuticle, and absence of
waterproofing in the epicuticle explains why these animals are
restricted to moist environments. If your specimen is preserved in
liquid you might want to let it dry off a bit before making your
observations, if it gets dry enough you may even feel the velvety
texture of the body wall!
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© JON G. HOUSEMAN
BIO 3323
Entomology
Figure 1 External
anatomy of the ventral
surface of Onycophora.©
Antenna
BIODIDAC
Oral
papilla
Labrum
Jaw
Lip
Prebuccal
lobes
Claw
Leg 1
Excretory
pore
Head
There is very little cephalisation in onychophorans and three pairs of
appendages mark the head's location. The most obvious are the
paired annulated antenna. These aren't segments! Look on the dorsal
surface near the base of the antennae and you'll see a pair of eyes.
The innervations of the antennae and eyes by the protocerebrum and
deuterocerebrum are typical of the other uniramians that have a
brain consisting of three parts. On the sides of the head are the oral
papillae that contain the slime glands that shoot sticky threads that
entangle prey and in some species harden to form a permanent trap.
The mouth opens on the ventral surface of the head and is
surrounded by peribuccal lobes. Look inside and you should be able
to see the third pair of appendages, the paired chitinous jaws. When
onychophorans feed they push their mouth against their meal and
lateral movements of the jaws tear off pieces of food that are mixed
with saliva liquifying the meal before its sucked back into the
digestive tract. The fleshy lobes surrounding the mouth make sure
that nothing leaks out.
Trunk
Just as there is no visible segmentation on the head, the same is true
of the trunk. The fleshy lobopods extending from the sides of the
animal are all this is visible of an underlying segmental plan that is
repeated internally by the metanephridia. The legs move using an
internal hydrostatic skeleton and are distinctly different from the
jointed legs of other arthropods. Even with this difference the
chitinous claw at the tip of the leg is similar to that found in
panarthropods and is used to grip the substrate when moving . Look
closely under the ventral surface of the legs where they connect with
the trunk and you'll see a groove in side of which is located the
opening to the metanephridia - you won't be able to see the opening
itself. You also won't be able to see the small spiracular openings in
the grooves of the rings that encircle the body. Look closely at the
body surface and you'll see that there are two types of tubercles and
although both are probably sensory some have a cuticular spine at
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Entomology
BIO 3323
their tip. At the posterior end of the animal is the anus, and
depending on the species available the gonopore opens between the
last two pairs of legs.
TARDIGRADA
Tardigrades are microscopic in size and only a few get any larger than
a millimeter or two in length. When they were first observed crawling
up small pieces of vegetation, how they used their four-paired legs to
paw the surface was reminiscent of bears pawing at their food.
Tardigrdes are all aquatic and found in both freshwater and marine
environments. The chitinous cuticle covering a tardigrade's body has
an outer epicuticle, formed of cross-linked proteins; middle
intracuticle containing lipid; and inner procuticle, a mixture of chitin
and proteins. The cuticle is not waterproofed. Tardigrades are
capable of withstanding adverse conditions by cryptobiosis. The
dormant forms, called tuns, drop to only 3 percent water and load up
with trehalose and glycerol to protect them from ice crystal
formation. The tuns can survive temperatures from -272 degrees C to
151 degrees C, live for over 100 years, and survive radiation levels
1,000 times what humans can handle. If you're using prepared slides
be sure to look at a number of different individuals. Some will be
squished making it impossible to see any structures, while others may
be on their sides, backs or fronts - take advantage of these different
orientations in your observations.
Figure 2 External
anatomy of the dorsal
surface of Tardigrada.©
Cirrus
BIODIDAC
Eyespot
Frontal plate
Scapular plate
Median plate
Leg
Paired plates
Lateral filament
Terminal plate
Claw
Papilla
Head
Cephalisation in these animals is weak, and there are no appendages
like those found in other panarthropods. They may have been lost as
these unique little animals adapted to their miniature world. The
mouth is located at the anterior tip of the animals. In prepared slides
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© JON G. HOUSEMAN
BIO 3323
Entomology
the stylets of the feeding apparatus should be visible through the
transparent cuticle. Tardigrades feed by piercing their food and
sucking the fluids out using the muscular pharynx that is also visible.
Trunk
The trunk consists of four segments covered with sculpted cuticular
plates. Four pairs of fleshy lobe-like legs include three that extend
from the sides of the tardigrade and the posterior most pair extending
behind. The lobe-like legs are extended by hydrostatic pressure and
retracted by bands of muscle that extend to the tip of each leg . The
legs terminate in a claw, or in some species an adhesive pad.
CHELICERATA - MEROSTOMATA
Don't let the common name of Limulus fool you. The horseshoe crab
isn't a crab it's a chelicerate, but a very ancient one at that. There is
one species that comes ashore on the east coast of the North America
and two more inhabit Asian oceans. These living fossils are all the
remains of an ancient chelicerate lineage and their bodies are
covered by a massive tough exoskeleton that protects the underlying
structures. They also burrow into the sand for further protection. The
body consists of the usual chelicerate prosoma and posterior
opisthosoma connected to each other by a hinge-like ligament. The
opisthosoma has a posterior spine, or telson, that is used to flip the
horseshoe crab over if it happens to end up on its back.
External anatomy
The body of all chelicerates is divided into two main tagmata, the
anterior prosoma (or cephalothorax) and the posterior opisthosoma
(or abdomen). The long tail isn't a segment, or tagma. Because it's
located behind the anal opening it's a telson. The cuticle of the
horseshoe crab is hardened by the calcium salts and in this way it is
similar to that of the Crustacea.
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Entomology
BIO 3323
Operculum
Gnatobases
Book
gills
Carapace
Figure 3 External
anatomy of the ventral
side of the horse show
crab.© BIODIDAC
Mouth
Telson
Anus
Chelicera
Pedipalp
Gill opercula
First
walking
leg
Abdomen
th
4 leg
Opisthosoma
Prosoma
Prosoma
The dorsal surface of the prosoma forms an enlarged horseshoe
shaped carapace that surrounds the legs underneath. The prosoma is
flattened and this helps the animal to shovel its way into the
sediments. At the outer edge of the lateral ridge on the dorsal surface
is a pair of compound eyes. Merostomata are one of the few
chelicerates that still have compound eyes but they may not be
homologous to the compound eyes that are found in the rest of the
Arthropods. On the anterior tip of the medial ridge are the median
eyes. These simple eyes may be hard to see if there is debris or
organic matter encrusted on the surface of the carapace.
On the ventral surface of the prosoma are the eight pairs of
appendages typical of all chelicerates. The chelicera are small, consist
of only three joints and are located in front of the mouth. Behind
them are the larger pedipalps which have a similar structure as the
following four pairs of legs behind them. In mature horseshoe crabs
the male pedipalp is modified and the last segment becoming thicker
and hook-like. It's used as a clasper to hold onto the carapace of the
female during mating . The tips of the legs are chelate and used to
manipulate food and pass it to the base of the legs where the basal
segments of each leg forms a spiny gnathobase used to grind and tear
food before it is passed to the mouth underneath. If your specimen
isn't too brittle try spreading the legs to see the mouth. Although
similar in segment number and the presence of the gnathobase the
last pair of legs have two unique modifications. On the base of the
leg , and on the outer surface, is a flattened, spatula-like structure that
is used to clean debris from the gill surface. The second modification
is found at the tip of the leg where the second to last segment consists
of four flattened plates that help to push the horseshoe crab is it
burrows. At the base of the last pair of legs is a moveable cuticular
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© JON G. HOUSEMAN
BIO 3323
Entomology
extension called the chilaria and are all that remains of the pregenital
segment of the opisthosoma.
Opisthosoma
Six flap-like plates are found on the ventral surface of the
opisthosoma. Lift up each and look at what is underneath. Under all
but the first you'll find the book gills that are used for gas exchange
and the flapping motion of the opercula pump water over the
respiratory surface. The first pair of plates is the genital opercula and
paired genital openings can be seen underneath along the midline
and about halfway back from the margin of the plate. In addition to
the six opercula the six flexible spines on the margins of the
opisthosoma identify six of the segments that form the tagma. The
anal opening is located at the base of the telson.
CHELICERATA - SCORPION
Like the horseshoe crab segmentation is still visible in the tagma of
the scorpion and this reveals how ancient these chelicerates are.
Scorpions were some of the first animals to invade the terrestrial
environment, feeding on soft-bodied invertebrate living in moist
locations. Chronologically then, they are the first arthropod on land
but it would be the insects that would be the most successful at
exploiting the environment and as their numbers grew they became
food for the scorpions
Figure 4 External
anatomy of the dorsal
surface of a scorpion. ©
BIODIDAC
Median
eyes
Prosoma
Chelicera
Pedipalp
Coxa
Mesosoma
Pretarsus
Tarsus
Tibia
Femur
Patella
Trochanter
Metasoma
Telson
Sting
External anatomy
Like other chelicerates a scorpion's body is divided into two tagma,
that anterior prosoma, or cephalothorax, and posterior opisthosoma,
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Entomology
BIO 3323
or abdomen with the opisthosoma being divided into an anterior
mesosoma and distal metasoma with the sting at it's tip.
Prosoma
A cuticular carapace covers the dorsal surface of the prosoma and at
the anterior end are the four pairs of simple eyes, ocelli. One pair
forms the larger medial eyes and the other three pairs of ocelli are
located on the lateral edge of the prosoma. Six pairs of appendages
are attached to the prosoma, the chelicera, pedipalps and four pairs
of walking legs. When viewed from the ventral surface the base of the
various appendages surround a small sternal plate. The hardened
pincers of the chelicera are attached to a large segment that forms the
top of the buccal cavity. The pincers, or chelae, are used to tear and
rip apart captured food before it is swallowed. The second pair of
appendages are the six-segmented pedipalps consisting of a basal
coax, trochanter, femur, patella, tibia and tarsus. The last two
segments of the pedipalps form the pincer-like chela used to capture
prey and the coxal plates form the sides of the buccal cavity. The
eight segmented walking legs are attached to the body by the coax
and additional segments include the trochanter, femur, patella, tibia,
metatarsus, tarsus, pretarsus and claws. Cuticular extension of the
coax on the first and second pairs of legs forms the bottom of the
buccal cavity.
Figure 5 External
anatomy of the ventral
surface of a scorpion.©
Pedipalp
BIODIDAC
Chelicera
Genital operculum
over genital pore
Pectines
Book lungs
Opithosoma
Each segment of the opisthosoma consists of a dorsal cuticular plate,
the tergite, connected by a pleural membrane to the ventral sternite.
The most conspicuous feature on the ventral surface of the mesosoma
are the paired pectines. They resemble a comb, with a series of
cuticular teeth that are embedded in a rod attached to the second
segment of the mesosoma. The pectines are have a rich nerve supply
that extends into each of the teeth suggesting a sensory role for the
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© JON G. HOUSEMAN
BIO 3323
Entomology
structure, although just what it detects is still not clear to zoologists.
In front of where the pectines are attached to the mesosoma, and on
the first mesosomal segment, are two cuticular plates, the genital
opercula, the cover the openings to the reproductive system. Both
the genital opercula and the pectines are modified appendages. The
remaining mesosomal segments have no appendages but on the
ventral surface of segments three to six are the spiracular openings to
the book lung .
The five segments of the metasoma consist of the fused cuticle of the
tergite and sternite, there is no pleural membrane, with the sting
located at its tip. The sting is not a segment and is also referred to as
the telson because of its location posterior to the anus. The sting
consists of a hallow bulb and barb and the poison located in the bulb
release their poison through an opening at the tip of the barb. The
anus is located in the membranous region between the sting and the
last metasomal segment. Most scorpions feed on insects and small
invertebrates and when prey is located they grasp it with the chelae
of the pedipalps and the opisthosoma is bent over the head and the
captured prey is stung . Food is torn off and passes in to the buccal
cavity, mixed with salivary secretions and digestive enzymes to
liquefy it, and strained by setal hairs on the gnathobases before it
enters the digestive tract.
CHELICERATA - SPIDER ARGIOPE
Nothing evokes fear and anguish in more people than spiders arachnophobia. Spiders are aerial predators of insects and have
mastered the terrestrial environment. They are covered with an
alpha-chitinous exoskeleton with a waxy epicuticle essential for
survival on land. The compound eye typical of other arthropods is
missing and instead they rely on tactile information from the webs
they spin and if vision is important they use their simple eyes. Instead
of mandibles they use chelicera as their main feeding appendage.
Spiders are variable in their morphology and the common garden
spider, Argiope sp., allows us to identify most of the characteristics of
animals in the group.
External anatomy
One of the things you'll immediately notice about your spider is that
its legs are positioned above the body, rather than underneath! In a
unique way that optimizes the small space inside the leg a spider legs
contain a single muscles that works against the elasticity of the leg
itself. Instead of having one muscle to raise the leg and another to
lower it, muscles move the leg down and the elasticity of the leg
moves it back up. When they die the muscles relax and the spring of
the leg raises it over the body. The spiders cuticle is leathery and
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Entomology
BIO 3323
their bodies aren't covered with the tough, thick cuticle typical of
insects and large Crustacea. The chelicerate body is composed of two
tagma, and anterior prosomoa, or cephalothorax, and posterior
opisthosoma, or abdomen.
Fang
Chelicera
Figure 6 External
anatomy of the ventral
side of a spider.©
BIODIDAC
Pedipalp
Prosoma
Pedicel
Book lung
Epigastric furrow
Opisthosoma
Spiracle
Anus
Spinnerets
Prosoma
A carapace covers the dorsal surface of the spider prosoma and at the
anterior end are four pairs of simple eyes. Although it may look like
there are only six, the most lateral "eye" is actually a combination of
two. Some of the eyes look forward, others up or down and to the
side. Like most chelicerates the compound eyes are missing , as is the
deuterocerebrum of the brain that would innervate it. The prosoma
has six pairs of appendages attached to it: the chelicerae, pedipalps
and four pairs of walking legs. The distal tips of the chelicerae are
modified into fangs attached to a large basal segment that is in turn
attached to the head. The poison glands, which produce a mixture of
digestive enzymes and neurotoxins, are located in the basal segment
and a duct carries its secretions to an opening at the tip of the fang .
Look at the head from the ventral surface and located the enlarged
base of the six-segmented pedipalp which helps to manipulate food
before it's ingested. Termed either a maxilla or gnathobase the latter
is probably the better term since it prevents confusion with the
maxillary appendage found in other arthropods. A labium is located
between the gnathobase of the pedipalps forming the lower lip of the
buccal cavity. In male spiders the pediplaps are modified for sperm
transfer. The upper lip, labrum, is located behind the chelicera and
may be hard to see. If your specimen isn't too brittle, open the buccal
cavity and try and locate the labrum inside.
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BIO 3323
Entomology
A large sternal plate covers the ventral surface of the prosoma.
Around its margins is the pleural membrane that connect the dorsal
carapace and ventral sternum. The pleural membrane is hard to see
because it surrounds the coxal joints of the four pairs of sevensegmented walking legs consisting of the coax, trochanter, femur,
patella, tibia, metatarsus, tarsus and its claws.
Opisthosoma
Although the membranous cuticle of a spider hides it, the
opisthosoma is composed of twelve segments consisting of dorsal
tergites connected by pleural membranes to ventral sternites. The
first segment of the opisthosoma is modified into the waist-like
pedicel that connects the opisthosoma to the prosoma. At the
anterior end of the opisthosoma are two triangular cuticular plates
that mark the position of the underlying book lungs that open to the
outside through the slits located just behind these plates. The book
lungs are formed from sheets of thin cuticle, lamellae, arranged like
the pages of a book, the origin of the structures name. In female
spiders the cuticular extension between the spiracular openings is the
epigynum that covers the gonopore and contains the opening to the
seminal receptacles.
At the posterior end of the opisthosoma are the three pairs of
spinnerets and the anal papillae. The two largest spinnerets are
composed of two segments and located towards the anterior and
posterior and surround the unsegmented middle spinnerets. To see
the middle spinneret you'll have to move the anterior and posterior
ones out of the way. The spinnerets origins are though to be the
appendages that were originally on these segments. Look closely and
you'll see a variety of bumps on the surface and each produces a
different type of silk from the silk glands underneath. Locate the anal
slit on the tip of the anal papillae. In front of the spinnerets is a single
medial spiracular opening . Spiders often have two types of
respiratory systems including the book lungs that we've already
mentioned and a tubular tracheal system. This spiracle is the opening
to the tracheal system.
UNIRAMIA - CHILOPODA
Chilopods are commonly referred to as centipedes, although in
reality there are never 100 pairs of legs - its usually somewhere
between 15-177 pairs. Centipedes are live in moist, terrestrial
environments and are predacious feeding on insects, worms, and
small molluscs. They can range in size from 10 centimetres to 30
centimetres found in some tropical species. Their outer epicuticle of
the cuticle lacks the waterproofing waxes found in other uniramians
and this, combined with the use of spiracles that can't be closed,
explains why these myriapods are restricted to moist environments.
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Entomology
Tergites
BIO 3323
Eye
Antenna
Figure 7 External
anatomy of head region
of a centipede. ©
BIODIDAC
Second maxilla
Labrum
First maxilla
Maxilliped (prehensor)
Second leg
First leg
External Anatomy
The body is divided into two tagma with the anterior trunk followed
by the posterior trunk. The trunk is dorsoventrally flattened and a
pair of legs extends from the sides of the segments. The first and last
two segments don't have legs.
Head
The head consists of the typical six segments that are found in all
uniramians and appendages include a pair of antennae, mandibles
and two pairs of maxillae. The antennae are formed from twelve or
more segments and vary in length depending on the species. As you
look at the ventral surface of the head the most obvious feeding
appendage is the poison claw, or fang . Technically it's a maxilliped
since it is a trunk appendage that is now involved in feeding and in
poisonous species it injects poisons from the tip of the claw to
subdue prey. In non-poisonous species it is still an affective way of
capturing prey. The mandible is also easy to see with the darkened
teeth. Between the mandible and the maxilliped are the two pairs of
maxillae. The first maxillae are fleshy lobes and the second maxillae
are fused along the midline to form the base of the bucal cavity and
only the second pair of maxillae has palps. Centipedes are nocturnal
and in most species the compound eye has disappeared and a cluster
of ocelli remain, although in some species this many include as many
as 200 simple eyes that may be confused with a compound eye.
Simple eyes don't form images and only respond to light levels.
Trunk
Each of the trunk segments is dorso-ventrally flattened and the dorsal
tergite is connected to the ventral sternite by a pleural membrane.
Seven segmented legs extend from the pleural membrane. As we've
mentioned already the first trunk segment is the maxilliped consisting
of four segments. At the tip is the hardened point of the claw; the
basal segments meet along the midline creating a second lower lip to
enclose the buccal cavity. At the posterior end of the body the
second to last segment has appendages modified as gonopods, or in
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BIO 3323
Entomology
some species these last appendages may be large and important
sensory structures. The last segment houses the opening to the
reproductive system and the anus. Look closely at the wall of each
segment and locate the spiracles. In some species they are located on
alternating segments.
UNIRAMIA - DIPLOPODA
The diplopods are commonly called millipedes, a name derived from
what seems to be thousands of legs that really number around 375
pairs. Like centipedes, millipedes are terrestrial and restricted to
moist soil, leaf litter and rotting vegetation because their epicuticle
lacks the waterproofing waxes and the spiracles can't be closed. The
diplopods and myriapods are combined as the Myriapoda. Unlike
their centipede cousins, millipedes aren't predators and are
herbivores feeding on plants or decomposing plant materials.
Figure 8 External
anatomy of head region
of a centipede. ©
Collum
Tergites
Eye
BIODIDAC
Sternites
Antenna
Labrum
Genital pore
Gnatochilarium
Mandible
External Anatomy
The body is hard and cyclindrical and composed of two tagma, a
head and trunk. While their common name comes from their
numbers of legs, Diplopoda refers to what appears to be two legs for
each segment of the trunk when seen from above. Well take a look at
this a little later when we examine the trunk in more detail.
Head
The head of a millipede is bent forward and the dorsal cuticle forms a
head capsule or epicranium. The anterior most edge of the head
capsule is bilobed and it's edge is ridged with teeth. It's assumed that
this is homologous with the labrum found in other uniramians and
forms the roof of the buccal cavity. Two pairs of segmented antennae
are the most prominent appendages on the head and they can be
held tight to the head in grooves. At the base of each antennae is a
cluster of approximately fourty simple eyes. The ventral surface of the
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Entomology
BIO 3323
head is formed by the gnathochilarium that is the floor of the buccal
cavity. The gnathochelarium are homologous with the first maxillae
found in other unirmaians but there is either no second maxillary
segment that appears to have been lost or the two maxillary segments
have fused to form the gnathochelarium. Zoologists still haven't
resolved this issue. Between the gnathochilarium and the anterior
margin of the head capsule are the mandibles. Use a pair of forceps
to pull the gnathochilarium back and expose the inner surface of the
mandible composed of two segments. A large, immovable basal
segment, and a distal segment modified as a tooth, scoop, bristled
plate and grinding molar surface.
Trunk
First examine the segmentation near the centre of the trunk where
each "segment "is an almost perfect circle with the dorsal tergite
forming most of the circle. The tergites overlap and the posterior
margin of each overlap the anterior margin of the one behind. This
overlap is important part of the defensive reactions of millipedes that
roll up in a tight ball with out exposing any of the fleshy membranes
between the tergites. A second defense is secretions of the
repugnatorial glands located at the base of the coax, although the
openings of these are too hard to see. On the ventral surface the two
pairs of legs for each of the apparent dorsal segments are easy to see
and the functional unit is referred to as a diplosegment, a fusion of
two ancestral segments. The legs are seven segmented with a basal
coax, trochanter, prefemur, femur, tibia, tarsus and the claw shaped
last segment. The first five segments of the trunk aren't
diplosegments. The first is the enlarged tergite of the collum, and
although we count it here as the first trunk segement there are some
zoologists that believe that this may be the second post-mandibular
appendage of the head. The second and first trunk segments don't
extend down the sides as far as those on the rest of the body and a
pleural membrane connects the tergite and sternite to each other.
The second tergite has a lateral process or spine the projects towards
the midline. By the third segment the tergites and sternites meet. In
each of the first five segments a single pair of legs correlates to a
tergal plate even though two pairs of legs are paired by their
proximity. Because there are five pairs of legs here and two in each of
the remaining diplosegments millipedes always have an odd number
of legs. Look closely at the lateral surface of each tergite and locate
the spiracular opening . If you're having trouble finding these on a
diplosegment they are often easier to see on the first five trunk
segments. The posterior end of the millipede consists of two plates
that meet along the midline forming the slit like anus. This last trunk
segment lacks appendages.
The male genital opening is located between the sternum and tergite
of the second segment. The penis is usually retracted and the medial
slit from which it everts is difficult to see. The most obvious male
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BIO 3323
Entomology
feature are the gonopods on the second full diplosegment. Here the
coax of the legs have been modified and embedded in a fleshy
cuticular membrane. The gonopods are probably modified legs and
have two segments consisting of an elaborate spoon shaped piece
and blade they function like the chela of other arthropods. During
mating the male bends its body so that the everted penis contacts the
gonopods and either sperm itself, or a spermatophore is formed and
passed to the female. The female genital opening is located in the
vulva, a almond shaped piece of cuticle located more to the side and
front edge of the second tergum - this may be hard to see. The vulva
stores sperm from the male, acting as a seminal receptacle, and
releases sperm to fertilize the eggs as they are laid.
CRUSTACEA
The Crustacea are the dominant Arthropod in the marine
environment and they range in size from the microscopic to the
gigantic. Obviously we can't go over all the different classes and types
here so we'll bring out our old friend the crayfish back and look for
the typical crustacean characters using that specimen.
Examine your specimen, and identify the key arthropod
characteristics such as the hardened outer cuticle and jointed
appendages and compound eyes, which in Crustacea are always
located on the ends of stalks. The body is arranged in a linear series
of metameres which have undergone tagmosis into distinct body
regions. The two principle regions, or tagma, in the decapods are the
cephalothorax and abdomen (pleon). The cephalothorax is covered
dorsally by the carapace and the division between the head and
thorax (pereon) is represented by the cervical groove in the carapace.
Three of the eight thoracopods have fused with the head and are now
referred to as maxillipeds. This leaves 5 pairs of appendages on the
thorax. Posterior to the cervical groove and running parallel to the
body axis are two branchial grooves which identify the underlying
branchial or gill chambers. The structural organisation of the
abdomen is simple with overlapping curved dorsal tergites and
ventral sternites connected by articulating membranes.
PAGE: 14
INSECT RELATIVES - PANARTHROPODA
© JON G. HOUSEMAN
Entomology
BIO 3323
Figure 9 Major external
features of a crayfish ©
Rostrum
Eye
Ce
lot
ph a
ax
ho r
Abdom
e
BIODIDAC
n
Uropods
Swimmerets
Telson
Antennule
Carapace
Antenna
Maxilliped
Cheliped
Walking
legs
In the ancestral Crustacea all appendages on each segment would
have been similar in appearance and when you look at the ventral
surface of your specimen you can see the serially homologous
appendages.all the appendages are built on a biramous plan
consisting of a protopodite, exopodite and endopodite but each of
these parts of the basic limb can be modified in a variety of different
ways. Pieces may be lost, enlarged, added or fused.as individual pairs
of appendages become specialised for the different roles of food
gathering , locomotion, respiration and we see the ancestral biramous
plan become obscured. Each function requires different
modifications and as a consequence the appearance of the
appendages changes, although serial homology is maintained. How
are the basic elements of the biramous appendage modified? Try and
keep track of what is what using diagrams.
Examine the specimen and identify appendages starting from the
anterior most antennules, antennae, followed by mandibles and the
two maxillae. These five cephalic appendages are found in all
Crustacea. Three maxillipeds follow and mark the division between
the head and thorax (pereon). The first of five walking legs or
pereopods is modified as a prehensile organ and referred to as a
cheliped and it is followed by the 4 pairs of walking legs. The walking
legs are biramous but only the endopodite remains. The tip of the
legs are chelate and you should be careful that you don't confuse this
chelate condition with being biramous. The difference here is that
the second to last segment extends a lateral process forward. It gives
a branched appearance but in fact the segments that make up the leg
are still in a linear sequence and the apparent branch is at the distal
end rather than at the base of the appendage. Behind the walking
legs are six pairs of abdominal appendages, five biramous
swimmerets and the last set of modified legs the uropod which, when
combined with the telson, forms the rudder like posterior end of the
PAGE: 15 - INSECT RELATIVES - PANARTHROPODA
© JON G. HOUSEMAN
BIO 3323
Entomology
animal. The first two swimmerets are sexually dimorphic and are
modified as copulatory organs in the male.
PAGE: 16
INSECT RELATIVES - PANARTHROPODA
© JON G. HOUSEMAN