FSE04
forensic ento mology
Insect Structure
Teacher background information
Classification is essentially a hierarchy that
progressively becomes more specific.
For example, a Phylum groups organisms based on
similarities in basic body plan and organization, an
Order often relates to the specialization of specific
limbs and body segments while a Species means that
all members can interbreed.
Classification of organisms is a fluid area of study.
As more information becomes available previous
classifications are often re-thought and depending
on what textbook is read there are often extra “sub”
groups such as sub-phylum, superclasses etc. It is a
complex field.
Table 1: Classification examples.
Classification
Humans
Western Goldenhaired Blow fly
Dermestid Beetle (skin beetle)
Domestic cat
Kingdom
Animalia
Animalia
Animalia
Animalia
Phylum
Chordata
Arthropoda
Arthropoda
Chordata
Sub-phylum
Verterata
Uniramia
Uniramia
Vertebrata
Class
Mammalia
Insecta
Insecta
Mammalia
Order
Primates
Diptera
Coleoptera
Carnivora
Family
Hominidae
Calliphoridae
Dermestidae
Felidae
Genus
Homo
Calliphora
Dermestes
Felis
Species
sapiens
albifrontalis
maculatrus
catus
Classes of the Phylum Arthropoda – jointed feet
Most arthropods are insects BUT there are also bugs
that are NOT insects such as spiders (Arachnida),
crayfish & woodlice (Crustacea) and millipedes &
centipedes (Diplopoda & Chilopoda - sometimes
collectively known as Myriapoda).
There are 11 classes in the phylum Arthropoda. ALL
arthropods have the following features:
•
are bilaterally symmetrical,
•
have segmented bodies with jointed appendages
(modified or lost),
•
have a hard exoskeleton,
•
have an open circulatory system, and
•
gaseous exchange takes place through the body
surface (gills, trachea, book lungs).
FSE04
forensic ento mology
Insect Structure
Table 2: Classes of arthropods
Sub-phylum
Class
Examples
Characteristics
Chelicerata
Merostomata
Horseshoe crabs, eurypterids
Large shield that covers the cephalothorax.
reduced compound eyes, second pair of
appendages resemble walking legs long, spike-like
appendage called a telson that projects from the
rear of their bodies. Respiration is via book gills.
Pycnogonida
Sea spiders
Small bodies and relatively long, hinged legs,
unusual proboscis (a chamber with an opening at
the distal end), body itself is not divisible.
Arachnida
Spiders, ticks, mites
One or two body parts, modified mouthparts, four
pairs of walking legs, most are terrestrial.
Remipedia
Nectiopoda – found in deep caves
connected to the sea
Have a long trunk made up of many segments,
each bearing a pair of biramous swimming
appendages.
Cephalocarida
Tiny marine animals living on the sea floor
Small compound eyes that are buried in the
exoskeleton, no abdominal appendages.
Branchiopoda
Fairy shrimp, water fleas
A small body (0.25 mm - 10 cm long), paired
compound eyes, single simple eye, simple mouth
parts, leaf-like appendages and minimal body
segments.
Maxillopoda
Ostracods, copepods, barnacles
Feed by means of their maxillae, basic plan of
5 head and 10 trunk segments followed by a
terminal telson. The abdominal segments typically
lack appendages.
Malacostraca
Krill, crabs, shrimp
Hard, calcified exoskeleton, body divided into
three main segments: cephalon, thorax, and
abdomen. The head and thorax are fused into a
cephalothorax, five segments in the head, eight in
the thorax, and six in the abdomen, each segment
bears a pair of appendages.
Chilopoda
Centipedes
Multiple flattened body segments with distinct
head, large antennae, chewing mouthparts,
poisonous fangs, one pair of legs per segment,
terrestrial, predatory: feed on insects.
Diplopoda
Millipedes
Multiple body segments with distinct head,
antennae, chewing mouthparts, two pairs of legs
per segment, breath via tracheae, terrestrial, feed
on plants (dead or living).
Insecta
Flies, ants, butterflies
Three body parts, antennae, large compound eyes,
mouthparts for chewing, sucking or sponging,
three pairs of legs, usually two pairs of wings,
breathe via tracheae, most are terrestrial.
Crustacea
Uniramia
Myers, P. 2001. “Arthropoda” (On-line), Animal Diversity Web. Accessed January 15, 2007 at
http://animaldiversity.ummz.umich.edu/site/accounts/information/Arthropoda.html
FSE04
forensic ento mology
Insect Structure
Entomology: the scientific study of insects.
Structure of insects
Insects are a class of arthropods that have been
incredibly successful.
Key structures of insects include:
•
three main body regions: head, thorax and
abdomen (these regions may be further
segmented),
•
an exoskeleton,
•
no more than three pairs of segmented legs
attached to the thorax,
Possible reasons for their success include:
The ability to fly.
•
Escape predators.
•
Change habitats.
ö
Find food.
•
one pair of antennae,
ö
Find mating partners.
•
compound eyes,
•
one or two pairs of wings, and
•
small holes or spiracles along the side of their
body.
Small size.
•
Can occupy and exist in a wide range of habitats.
•
The exoskeleton provides great protection and
prevents water loss.
Diverse body structures.
•
Body structures enable insects to live in a wide
range of habitats. Eg, aquatic insects can extract
oxygen from water.
•
The process of metamorphosis enables insects to
exploit different food sources at different stages of
development.
Insects are the only arthropod class to achieve flight.
Food supply.
•
Insects feed on anything ranging from nectar to
dead flesh.
Reproduction.
•
They reproduce very quickly and in large numbers.
Insects also contribute significantly to the ecosystem.
Even though it would be great to be outdoors without
mosquitoes, a world without insects would be vastly
different.
For example, insects are:
•
pollinators,
•
a food source for many other animals,
•
decomposers and recyclers of natural materials,
•
producers of products such as silk and honey,
•
key members of medical and scientific research,
and
•
prime evidence in forensic science.
Figure 1: A diagram of an ant showing basic insect
structure.
USDA Forest Service Archives, USDA Forest Service, www.insectimages.org
FSE04
forensic ento mology
Insect Structure
Figure 2: A diagram clearly showing the segmented legs and wings originating from the thorax of an insect.
USDA ARS Archives, USDA Agricultural Research Service, www.insectimages.org
Websites
The following websites have some useful information.
The CSIRO site is extensive but does required some searching – a lot of information about insects is available.
http://www.ento.csiro.au/about_insects/index.html
Within the CSIRO site is the “Anatomical Atlas of Flies”. This is a fantastic online tool where students can view and
get information about the external structure and morphology of flies.
http://www.ento.csiro.au/biology/fly/fly.html
The Manduca Project is from the University of Arizona and is definitely worth a look. It has a substantial number of
resources including videos showing events such as moths emerging from their pupal case. The website is based
around students raising the tobacco hornworm Manduca sexta.
http://insected.arizona.edu/manduca/default.html
The Australian Museum site.
http://www.amonline.net.au/insects/insects/index.htm
A more “kid” orientated site but has lots of hidden information.
http://www.earthlife.net/insects/six.html
A useful site with general information.
http://www.kendall-bioresearch.co.uk/morph.htm
FSE04
forensic ento mology
Insect Structure
Insect Life cycle
The typical life cycle of an insect is as shown in Figure 3.
Figure 3: The life cycle of a cottonwood leaf beetle
(Chrysomela scripta).
Adapted from James Solomon, USDA Forest Service, www.
insectimages.org
For example, in Figure 4, various stages of the life
cycle of a cottonwood leaf beetle (Chrysomela scripta)
are shown on one leaf!
Figure 4: The life cycle of a cotton leaf beetle
James Solomon, USDA Forest Service, www.insectimages.org
FSE04
forensic ento mology
Insect Structure
Length of the life cycle.
The progression through the life cycle is dependent
on a host of variables with the main determinant
being temperature. This is discussed in detail in FSE07
– Forensic Entomology.
Metamorphosis
Metamorphosis refers to a major change of form or
structure during development. Most insects have a
form of metamorphosis. This strategy enables insects
to exploit different food sources at different stages of
their development. It is one of the key reasons for their
great success.
The Australian Museum:
http://www.amonline.net.au/insects/insects/metamorphosis.htm
An example of the absolute difference between
habitat and food source at different stages in the life
cycle is a mosquito. The larvae live in water and eat
plankton, the adult fly around and sucks blood! The
‘role’ of the larvae is to eat while the ‘role’ of the adult
is to reproduce.
Complete metamorphosis
The adult insect looks completely different to the
immature insect. As shown in Figure 3 and 4, the
insects have a life cycle where they completely change
from an egg → larvae → inactive pupae→ adult.
These insects are often called ‘Holometabolous’,
meaning they undergo a complete (Holo = total)
change (metabolous = metamorphosis or change).
For example, the pecan leaf casebearer (Lepidoptera
Acrobasis juglandis) undergoes complete
metamorphosis.
Figure 5: Complete metamorphosis in the pecan leaf casebearer.
Citation: Jerry A. Payne, USDA Agricultural Research Service, www.insectimages.org
FSE04
forensic ento mology
Insect Structure
Table 4: Major orders of insects with complete metamorphosis
Order
Example
Characteristics
Coleoptera
Beetles
The forewings are hardened into “elytra” which protect the delicate
hindwings. They have biting and chewing mouthparts.
Lepidoptera
Moths, butterflies
Two pairs of membranous wings and sucking mouthparts. At rest moths
hold their wings flat while butterflies hold wings vertically (and have
clubbed antennae).
Hymenoptera
Wasps, ants and bees
Membranous wings although sometimes wingless. Chewing mouthparts
and a constriction between abdomen segments: wasp waist.
Diptera
Flies
The hind wings are reduced to balancing organs called halters. Lapping,
sucking or piercing mouthparts.
Incomplete metamorphosis
Some insects however undergo incomplete
metamorphosis in that they gradually change to an
adult. Immature insects in these orders are called
nymphs and they gradually grow in size. This is called
Hemimetabolous or incomplete metamorphosis.
Figure 6: Incomplete metatmorphosis life cycle of the
brown marmorated stink bug (Halyomorpha halys).
Adapted from Citation: Gary Bernon, USDA APHIS, www.insectimages.org
FSE04
forensic ento mology
Insect Structure
Figure 7: Incomplete metamorphosis in the brown
marmorated stink bug (Halyomorpha halys).
Citation: Gary Bernon, USDA APHIS, www.insectimages.org
Figure 7 shows several early instar nymphs in three
different stages on an egg mass: the one dark red
nymph is a first stage or instar nymph, the light red
nymph has just molted and is therefore a new second
stage or second instar nymph. The other 4 black
nymphs are second stage or second instar nymphs. It
is at this stage when aggregating nymphs may have a
tick-like appearance. Note also that all the eggs have
hatched.
NB: the CSIRO site has lots of good information about
various orders of insects.
http://www.ento.csiro.au/education/insects/diptera.html
http://www.ento.csiro.au/education/insects/
lepidoptera.html
Substitute the name of the order of insect (just before
the html) in the above URL.
FSE04
forensic ento mology
Insect Structure
Table 3: Major orders of insects with incomplete metamorphosis.
Order
Example
Characteristics
Collembola
Springtails
Primitive wingless insects, largely in soils
Thysanura
Silverfish
Primitive wingless insects, in caves and damp houses
Ephemeroptera
Mayflies
Larvae in fresh water, adults live only days
Odonata
Dragonflies
Generally large insects found worldwide, carnivorous and herbivores, Larvae predators in
fresh water
Plecoptera
Stoneflies
Adults usually live near river banks, Larvae in fresh water
Blattodea
Cockroaches
Omnivorous, scavengers, found worldwide
Isoptera
Termites
Social insects living in vast colonies with one queen to lay all the eggs, most feed on wood
Mantodea
Mantids
Predatory insects with large eyes and grasping front legs, mostly in the tropics
Dermaptera
Earwigs
Omnivorous insects with fan-shaped hind wings and pincers on tail
Orthoptera
Grasshoppers
Grass-feeding insects with jumping hind legs
Phasmatodea
Leaf-insects
Leaf-feeding insects with camouflaged, flattened or slender bodies, look like leaves or sticks of
plants
Psocoptera
Book lice
Small chewing insects, feed on tree bark, book bindings, also found in food
Phthiraptera
Parasitic lice
Parasites of birds and mammals, live on skin, feed on feathers, skin or blood, wingless
Hemiptera
True bugs
Piercing or sucking mouthparts, feed on plants, insects or mammals
Thysanoptera
Thrips
Tiny insects with fringed wings, herbivorous with sucking mouthparts
Megaloptera
Alderflies
Larvae are aquatic and carnivorous, adults have long antennae, two pairs of large wings
Neuroptera
Lacewings
Predators as larvae, adults are carnivorous or herbivores
NB – not all insect orders are listed.
FSE04
forensic ento mology
Insect Structure
Glossary
Chrysalis: The pupa of a butterfly or moth.
Cocoon:
A silk casing within which larvae (particularly of some Lepidoptera) passes into and through
the pupal stage.
Compound eye:
A large grouping of light gathering organs that allow the insect to detect color and motion.
For most insects, resolution is not very good.
Frass:
Another term for larvae insect droppings or excretion.
Elytron:
The hardened protective forewings of a beetle.
Haltere:
Club shaped organ that take the place of the hind wings in true flies (diptera). They are used
for balance and stability in flight.
Hemimetabolism:
An incomplete metamorphosis where an insects development does not include a pupal
stage. The larvae are known as nymphs and resemble the adults.
Holometabolism:
A complete metamorphosis where an insects development passed through fours stages
- egg, larvae, pupae, & adult.
Instar:
The stage between each moult of a larvae. For example a first instar larvae would refer to a
larvae between hatching and it’s first moult. Final instar would refer to the last stage prior to
becoming a pupae.
Larvae:
The immature form of an insect that follows the egg stage.
Moult:
To shed periodically part or all of the outer covering which is then replaced by new growth.
Nymph:
A larvae of an exopterygote (insect with incomplete metamorphosis)
Pupae:
A non-feeding stage of development between the last larvae and adult forms, characterized
by many anatomical changes and, often, by enclosure in a cell or cocoon.
Tagmata:
The 3 main regions of the insect body: head, thorax and abdomen.
Ovipositor:
The organ with which many insects deposit their eggs. Some ichneumon flies have a long
ovipositor fitted to pierce the eggs or larvae of other insects, in order to lay their own eggs
within the same.
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