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Entomology
BIO 3323
EXTERNAL ANATOMY
An outer exoskeleton of hard, articulating cuticular plates is a feature
shared by all arthropods. It’s a complex non-living structure formed
from strengthening alpha-chitin embedded in proteins, the
procuticle, and topped with a waterproof epicuticle of crosslinked
proteins and waxes. The cuticle is secreted by the underlying , living
epidermal layer. This outer suit of armour creates a variety of
different problems for insects not the least of which, is that in order
to grow, the insect must escape the old cuticle and then lay down a
new one, it moults.
What’s interesting about the cuticle is the different shapes and forms
it has. The delicate wings of a small fly, the massive jaws of some
beetles, and the feathery antennae of moths all are made of cuticle. It
is the bioplastic of the living world and the plasticity of this outer
body covering helped to insure the success of insects. As you look at
the different external features on the animals during today's lab
always remember they are all made with the same cuticular
components.
INSECT MORPHOLOGY
The best way to appreciate the field of Insect Morphology is by
approaching it from a comparative standpoint. In the lectures we
have discussed the tremendous diversity that occurs within the Class
Insecta and how this is reflected in the diverse and multiple numbers
of environmental niches that these animals are able to occupy.
Underlying all of these adaptations are certain morphological, or
structural characteristics, that all insects share. In different insects
These have been modified into different structures designed for a
variety of different functions.
The terminology associated with the external anatomy is unique to
insect morphology and these terms are important as reference points
for subsequent discussions in both the laboratory and lectures. The
stuructures are important in insect identification, so when the keys
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Entomology
ask about a modification or shape of the paranotal lob of the
mesotharacic segment you’re going to need to know just what that
means.
External Anatomy - The lubber grasshopper
The Lubber Grasshopper will be used to introduce the external
characteristics of insects. Traditionally the grasshopper is used for this
type of laboratory exercise because it has a number of primitive
characters. You should be aware that although this animal represents
some ancestral characteristics it is, in its own right, a very specialised
animal, it’s jumping legs for example. We’ll look at modifications of
the basic body plan in the last part of the lab and with demonstration
materials as the term progreses.
Figure 1 Major external
features of the lubber
grasshopper. © BIODIDAC
Head Thorax
Antenna
Ocelli
Compound
eye
MesoPro-
Abdomen
Meta-
Femur
Tibia
Forewing
Hindwing
Cercus
Spiracles
Identify the three major tagma of an insect: The head, thorax and
abdomen. The body is covered by the exoskeleton or cuticle that is
laid down by the underlying epidermis. The exoskeleton is divided
into a number of plates, or sclerites, that can be separated from each
other by either sutures or membranes. The result is that different
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Entomology
BIO 3323
regions of hardened cuticle can bend against each other. What may
appear as lines between segments are often the inflection of the
cuticle to form internal skeletal elements such as apodemes.
Head
Vertex
Coronal suture
Epistomal
suture
Anterior
tentoral pit
Clypeus
Labrum
Mandible
Ocellus
Figure 2 Major external
features of the head of
the lubber grasshopper. ©
BIODIDAC
Gena
Frontal suture
Frons
Subgenal suture
The insect head is composed of six segments three of which are
preoral (pregnathal) and three post oral (postgnathal). (Remember
though that there is another school of thought that believes the insect
head had 5 segments ancestrally.) The last three segments have
appendages, the mandible, maxillae and fused labium; antennae and
labrum are preoral appendages. With the exception of the most
posterior lines, sutures, of the head there is no visible marker for the
underlying segmentation of the tagma. Most of the visible suture lines
result from inflections of the cuticle acting as strengthening ridges for
the head capsule. These internal inflections are called apodemes.
Identify the main features of the insect head. Compound eyes are
located laterally and a close examination will reveal the complex
pattern of ommatidia, the fundamental repeating optic units of the
eye. In addition to the compound eye, ocelli are located on the facial
region and serve a light receptive structures usually monitering light
levels. Insects may have up to three ocelli and in your grasshopper
they may be hard to see if the specimen is still wet with the
preservative. The single pair of antennae are characteristic of the
Tracheata, and are composed of three basic parts; the most proximal
is the scape, the pedicel is next and the most distal part is the
flagellum. The latter is composed of a number of annulations and is
often elaborately modified in different insects. What is the difference
between an annulation and a segment? Examine the antennae closely
using the dissecting microscope and observe the sensory or setal
hairs. These have a primarily chemosensory role and may not be fully
apparent until the preservative has dried off the surface.
The head is divided into a number of regions by grooves or sutures.
The epistomal suture, for example, lies between the frons and the
clypeus and reinforces the head against the forces generated by the
mandibular musculature. While we refer to insects as having an
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Entomology
external skeleton that are points were the cuticle expands internally,
for example apodemes, to increase the surface area available for
muscle attachment. The small pits in this suture are the inflections of
cuticle that form the endoskelelatal tentorium of the head. The other
apodemes of the tentorium is located at the back of the head. Here
the occiput and postocciput are all the reamins of the heads original
segmentation and near the base of the plates is the second apodeme
of the tentorium. This very specialised piece of internal skeleton is
required for the muscles that move the mouthparts. A grasshopper
head that has bean cleared (the tissues are dissolved), by boiling it in
alkali shows this internal element. The inverted Y located on the head
is an ecdysial line, or suture, which splits open when the insect
moults. Familiarise yourself with the main plates of the head which
include the vertex, frons, gena, occiput, postocciput, and the various
sutures..
Figure 3 Major external
features of the head of
the lubber grasshopper. ©
BIODIDAC
Ocular suture
Gena
Ocellus
Antenna
Occipital suture
Postoccipital suture
Subocular suture
Occiput
Frons
Postocciput
Cervex
Posterior
tentorial pit
Post gena
Labium
Clypeus
Subgenal suture
Labrum
Mandible
Maxilla
The opening to the digestive system lies at the base of a buccal cavity
consisting of the upper labrum; the labial bottom; and the
mandibular and maxillary sides. The grasshopper mouthparts are
similar to the generalised, or primitive form. Carefully remove the
mouthparts by pulling the head forward from the thorax and cut
thorough the membranous neck region. The labrum is suspended
from the clypeus and forms the upper lip, or roof of the buccal cavity.
The mandibles are highly sclerotized and hardened with both a
cutting , incisor, and grinding , molar, region, locate both. The two
points where the mandible articulates with the head should also be
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Entomology
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obvious. The maxillae are also paired and composed of a tooth-like
lacinea, a galea and a sensory leg-like maxillary palp which is used to
taste the food.
Figure 4 Mouthparts of
the lubber grasshopper. ©
Clypeus
BIODIDAC
Labrum
Grinding
Mandible
Cutting
Mentum
Hypopharynx
Cardo
Stipes
Prementum
Paraglossa
Labial suture
Glossa
Palpifer
Palpus
Lacinia
Ligula
Labium
Galea
Palpus
Maxilla
This sensory structure is also covered with sensory hair similar to
those on the antennae. The labium represents two fused mouthpart
appendages and forms the bottom to floor to the buccal cavity. Again
sensory labial palps are present. The hypopharynx is not an
appendage but lies in the buccal cavity as a tongue like structure with
oral opening above and salivary opening below. As we will see later in
the lab this plan can be extensively modified and has allowed the
insects to feed on a tremendous variety of different foods.
Thorax
The thorax is locomotory in function and is composed of three
segments, the prothorax, mesothorax and metathorax. Each of these
thoracic segments bears a pair of appendages and, if present, wings
are on the mesothoracic and metathoracic segments. The thoracic
segments can be divided into three major sclerites which include the
dorsal notum (tergite), lateral pleura and the ventrally located
sternum. The movements of these three plates relative to each other
are important in understanding the flight mechanism. In most insects
the lateral pleura have fused with the ventral sternites to form the
bottom three sides to the thoracic box. The notum is connected by a
membranous part of the pleura to the box, forming the top. As the
notum rises the wings are lowered, as it falls, the wings are raised.
Look closely at the pleural plates of those with wings. A central
pleural suture divides the plural sclerite into an anterior episternum
and posterior epimeron. The inflextion of the suture braces the
thoracic box against the muscular forces of the flight muscles. Locate
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Entomology
the basalare and subalare sclerites in the pleural membrane under the
wing . These two scerites and the pleural wing process, the extension
of the pleura sclerite between them, are important in wing
movement.
Two pairs of spiracular openings are located on the thorax and these
regulate air intake into the internal tracheal system which is also an
unusual feature of insects that oxygenate their tissues by the direct
supply of air rather then by using a circulating respiratory pigment.
Locate the spiracles and the auditory tympanum.
The pleural sclerite also articulates with the leg which is attached in a
membranous area below the pleural suture. The insect leg is
composed of six segments and the tarsus is further divided into up to
five segments, and pretarsus with its claws. The divisions of the tarsus
gives the distal end of the appendage an even more multisegmented
appearance. Don't confuse these with the true leg segments. Later in
the course you’ll be counting tarsal and prestarsal segments and
you’ll need to know the difference! The tubular exoskeleton means
that only linear or planar articulations occur. Try to flex each of the
segments. Do they all move in the same direction relative to each other?
Identify the major leg segments include: coxa, trochanter, femur, tibia
and tarsus and the last pretarsal segment which has a claw. A fleshy
pad is often associated with the claw at the tip of the organism. This
pad when located between the claws is referred to as an arolium
while when located under and at the base of the claw it is called a
pulvilli.
Figure 5 Parts of the typical insect leg. © BIODI-
DAC
Tarsomere
Claw
Trochanter
Coxa
Tarsus
Tibia
Femur
The wings of an insect are often membranous and strengthened by a
series of ridges or veins. This arrangement of veins and cross veins is
an important taxonomic tool for identification of insects. Identify the
following wing veins costa, cubitus, subcosta, radial, medial, cubital
and anal. Are all of these veins visible on the grasshopper wing?
Abdomen
The apparent or visible segmentation of the insect abdomen varies in
different insect groups and but primitively this tagma was composed
of 11 segments. A typical abdominal segment consists of a sclerotized
tergite and sternite and membranous pleura. Spiracular openings are
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Entomology
BIO 3323
located on each of the segments and the terminal segments bear
appendages involved in copulation and egg laying . Identify the cerci
and ovipositor.
VARIATIONS ON THE BASIC PLAN
The insect head
Insect heads may have one of three different orientations based on
the position of the mouthparts relative to the remainder of the head
capsule. When the mouthparts are directed downward the head is
considered hypognathous compared to prognathous, when the
mouthparts are directed forward. In both these last two cases the oral
appendages are located anteriorly but when positioned in the
posterior region the head is opisthognathous.
Figure 6 Three different
types of insect heads
From left to right, hypognathous,
prognathous
and opisthognathous ©
BIODIDAC
There are some distinct advantages to these different orientations.
Many larval insects have prognathous heads because they live in the
same medium on which they feed. Forward directed mouthparts are
an obvious advantage. Take a look at the mealworm larva as our
example of a prognathous head. Others such as the cicada and a
variety of plant sap feeding insects must have the mouthparts
penetrate the vascular plant tissue. The achieve this requires
considerable strength and the opisthognathous head allows the insect
to apply the pressure required to penetrate the plant. Identify the
different mouth part types on the specimens.
Insect Mouthparts
One reason for insect success is their ability to exploit a wide variety
of food sources and their mouthparts have been extensively modified
to accomplish this. These modifications included changing the shape
of the of the various mouthparts and even the complete loss of some
parts are whole mouthparts. Identify the components in the different
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mouthpart types by using following examples (Text fig . 1-5) and
compare the various types to the descriptions provided in your text
book.
Grasshopper
Chewing mouthparts are the least specialised and the grasshopper is
an example of this type. Take a second look at the mouthparts from
your specimen and the prepared slides. Be sure to identify the
components so that you can keep track of them in the remaining
specimens.
House fly
Sponging mouthparts. are found in the adults of some of the higher
Diptera such as the housefly, Musca domestica. In these animals the
labium is enlarged in a fleshy labellum with numerous small channels
which by their capillary action draw up the fluids during feeding .
These are called canaliculi. The labium and labrum form the food
channel up which the liquefied food moves. The salivary canal is
separate from the food canal and allows the salivary secretions to be
pumped out while the ingested food passes into the organisms.
Mandibles have disappeared in the flies and only the palps are the
only maxillary features that remain.
Figure
7
Sponging
mouthparts of the Housefly. © BIODIDAC
Compound
eye
Antenna
Maxillary palp
Labrum
Labium
Food canal
Labium
Hypopharynx
Salivary canal
Labium
Labellum
Stable fly
Stable flies evolved from this same group of insects and had to modify
the basic plan of the sponging mouthpart into piercing structure that
allows them to obtain their blood meal. To do this the sponging
labellum becomes hardened and only its tip, with the teeth like
structures, remains flexible. Stable fly, Stomoxys calcitrans,
mouthparts are available on a prepared slide. How does labial
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Entomology
BIO 3323
modification allow for penetration through the skin in order to obtain the
blood meal? Compare the stable fly to the tsetse fly that you’ll be
looking at later in the lab.
Compound
eye
Antenna
Figure 8 Piercing mouthparts of the stable fly. ©
Maxillary palp
BIODIDAC
Labrum
Labrum
Food canal
Salvary canal
Maxillary
palp
Labium
Hypopharynx
Hypopharynx
Labium
Labellum
Labellum
Compound
eye
Antenna
Figure 9 Mouthparts of
the mosquito. © BIODI-
Food canal
Labrum
DAC
Mandible
Hypopharynx
Maxilla
Salivary canal
Labium
Maxillary
palp
Mandible
Maxilla
Labrum
Labium
Hypopharynx
Mosquito
Piercing-sucking mouthparts are seen in insects such as mosquitoes,
which represent the Diptera, and Hemiptera, such as Anasa tristis. In
one case these mouthparts pierce skin and the other plant seeds.
Look under the scope at the needle like maxilla and mandibles of
these two different species.
Butterfly
Siphoning mouthparts are characteristic of Lepidoptera or butterflies.
In this case the major part of the mouthpart is the galea and
mandibles and labium are lost.
Figure 10 Siphoning
mouthparts of a butterfly.
© BIODIDAC
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BIO 3323
Entomology
Antenna
Compound eye
Labrum
Labial palp
Maxillae
Maxillae
Nectar
channel
Horse fly
Horse flies, Tabanus sp., and deer flies, Crysops sp., are more
primitive dipterans than are the house fly and stable fly that you’ve
already looked at. In these flies all of the ancestral mouthparts are
still present and the horseflies use them in a very different way. The
mandibles and maxilla resemble scissor blades and when the fly feeds
it opens and closes these two sets of scissors cutting the skin surface.
Blood pools inside the ragged wound and the fleshy labellum, with its
sponging tip, sucks up the pooling blood. You’ll begin to understand
why a horse fly bite is so painful!
Figure 11 Mouthparts of
horseflies or deerflies.
Compound eye
Antenna
Maxillary palp
Labrum
Food canal
Mandible
Hypopharynx
Maxilla
Labium
Hypopharynx
Maxilla
Labium
Labella
Mandible
Labrum
Tsetse fly
The tsetse fly is a very important medical insect in Africa and it
transmits African sleeping sickness. Tsetse’s also transmit a similar
version of the disease to domestic cattle and its because of that large
parts of the African plains are unavailable for agriculture (Depending
on which side of the fence you sit, conservation vs. development, this
is either a good or bad thing). Compare the mouthparts of the Tsetse
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Entomology
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fly Glossina to those of the stable fly. How similar are they? Both of
these flies are advanced Diptera and they have only the labellum and
labrum and the hypopharynx inside to penetrate the host’s skin and
ingest the bloodmeal.
“Bugs” - Hemiptera
The Hemiptera are known as the true bugs and the whole order
specializes in piercing into their food source and sucking out the
liquids. Originally that involved plants and the mouthparts worked
their way through the plant tissues and tapped into the vessels that
moved fluid through the plant. With the mouthparts anchored into
these plant tubes the bug simply sucked up it’s meal. There are also
blood feeding bugs and it’s assumed that over time some of these
plant feeders probably pierced through the cuticle of other insects to
feed and along with that became predacious. From there it was
piercing vertebrates and drinking blood.
Figure 12 Mouthparts of
an hemipteran.
Compound eye
Labrum
Maxilla
Mandible
Maxilla
Food canal
Salivary canal
Labium
Labium
Mandibles
Antenna
Proboscis
We have the plant bug Anasa tristis for you to take a look at. The
maxilla form interlocking stylets with the mandibles on either side.
The mandible dig in and then anchor in place and then the maxillary
stylets are pushed forward looking for a vessel to pierce. If there isn’t
one they then anchor in place the mandible move forward to dig a
little deeper. The process repeats itself until a meal source has been
located. Both the mandibles and maxilla are wrapped inside the
labium to form the proboscis
Fleas
Adult fleas are unusual in that they have lost their wings and become
ectoparasitic on vertebrate hosts. The mandible is completely missing
and most of the work in feeding is done by the maxilla instead. The
lacinia of the maxilla do the piercing and cutting while what were the
galia form fleshy lobes. The maxillary lacinia combine with the
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Entomology
epipharynx (there is some debate as to whether this is the
hypopharynx or a new cuticular extension of the buccal cavity) and
surrounded by the labial palps to form the food canal.
Figure 13 Mouthparts of
a flea.
Antenna
Ocellus
Maxillary palps
Maxilla
Labial palps
Epipharynx
Maxillary lacinia
Honey bee
In the honey bee one ofthe biggest differences from that of other
insects is the labium and maxilla are enlarged. The central glossa of
the labium is modified into a curved tongue. The bee extends the
“tongue” and it becomes covered in nectar. After that it’s pulled back
into the surrounding tube that is created by the two maxilla and the
labial palps. From there the ingested nectar is sucked up and into the
digestive system. The mandibles are still present and used for eating
pollen and for work that is done on the hive which is essentially built
using the mandibles. When the mandibles are used the other
mouthparts are folded back and out of the way.
Figure 14 Mouthparts of
a bee.
Compound Eye
Antenna
Labrum
Mandible
Maxillary palp
Maxilla
Labial palp
Labium
Dragon fly nymph
Dragon fly nymphs are deadly predator in freshwater lakes and
streams the whole labium has been modified into a viscous
prehensile structure with opposable labial palps at the tip that grap
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Entomology
BIO 3323
onto the prey. The overlapping visual field of the large compound
eyes is located precisely at the point of full extension of the labium.
When a potential meal wanders into this field of view, the mouthparts
shoot forward and immediately retract carrying the trapped prey to
the mouth.
Figure 15 Mouthparts of
an Dragon fly nymph.
Labrum
Compound eye
Prementum
(Labium)
Prementum
(Labium)
Maxilla
Hypopharynx
Labrum
Postmentum
(Labium)
Prementum
(Labium)
Labial palp
Legs
Just as the mouthparts can be extensively modified so can insect legs.
The basic insect is used for walking and this type is the primitive or
ancestral form of the appendages. In many insects all three legs or
only certain pairs are modified in different ways so that they can
perform specialised functions such as swimming , climbing , jumping ,
digging and gathering specialised foods.
The femur and tibia in running legs (cursorial) are long and thin. The
increased length of the appendage means that the lever like motion
of arthropod movement causes the distal end to move over a greater
distance for the same amount of muscle movement. Jumping legs
(saltatorial) have large femurs which contain the enlarged muscle
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Entomology
required to extend the tibia. In jumping legs the femur and tibia are
close to each at rest and the tarsal claws are often well developed so
that the insect can get a good grip on the substrate when jumping .
Raptorial legs are specially modified to grasp and retain prey. The
classic example of this type of leg is the front leg of the preying
mantid. The inner surfaces of the tibia and femur in raptorial legs are
often equipped with sharp spines to help crush and immobilise the
captured prey.
Swimming (Natatorial) legs are modified to increase the available
surface that pulls against the water during the swimming stroke. This
can be achieved in one of two ways. Either the segments of the leg
are flattened or are fringed with flexible hairs
Digging legs are short and hardened and often flattened into shovel
like shapes. They may also have heavy toothed projections that also
assist in moving the soil.
Male water beetles are faced with a unique problem and their front
legs have been modified with suction cup like suckers so that they
can remain attached to the female during copulation.
WINGS
The major taxa within the class Insecta are characterised by the presence or absence of wings and their structure and venation when
present. The wingless insects are the Apterygota and the winged
forms the Pterygota. The most primitive wings are found, as the name
implies, in the Paleoptera with the rest of the winged insects being in
the Division Neoptera. Paleopteran insects include the Odonata, the
dragonflies and damselflies, and possibly the Ephemeroptera, the
mayflies or shad flies as they are called locally. Entomologists working
on Insect Phylogeny sometimes separate the two but we'll consider
both as Paleoptera. Insects in the Odonata and Ephemeroptera are
unable to fold their wings back and over the abdomen because the
axillary sclerites that allow this in other insects remain fused to the
wing veins. The result is that at rest a dragonfly's wings stick out from
the side and damselflies and ephemoropteran wings are held above
the abdomen. Some of the Neoptera also hold their wings in a similar
manner but the difference is that either the forewing , or most often
the hind wing , has additional folds not seen in the Paleoptera or the
wings surround or partially cover the abdomen. Although it's not
unique to them the large numbers of cross veins are also a good way
to identify the Paleoptera. Dragonflies and damselflies are available
for you to examine.
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Entomology
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Figure 16 Ephemeroptera are often included in
the Paleoptera and they
hold their unfolded wings
high above the body - a
characteristic of the Paleoptera
Within the Neoptera there are a variety of different ways that wings
are modified and, as you'll see when we start identifying insects, not
only the folding , but the similarities and/or differences between the
forewing and hindwing and their venation are important taxonomic
tools. Stoneflies, Plecoptera, have lots of cross veins in their wings
but look closely at how they are positioned over top of the abdomen.
Take a closer look at the hind wing and you'll see out it unfolds like a
fan. This difference in shape is one of the ways that insect forewings
may have a different appearance than the hindwing .
Figure 17 Stoneflies
have lots of cross-veins in
their wings this and their
mandibualte appearance
shouldn’t confuse you.
The hindwings fold and
these are neopterans. ©
BIODIDAC
The Orthoptera, which includes crickets and grasshoppers among
others, have leathery forewings compared to the membranous hindwings folded underneath. With your first look at the grasshoppers you
might disagree but look closely that hind wing is folded on itself a
number of times. Unfold it to see the difference in texture between
the two wings, another way wing pairs can differ from each other.
The Hemiptera, or true bugs, also have forewings that differ from the
hind wings. In this case the forewing as thicker at it's base and membranous at the overlapping tips. The half hardened wing is the origin
of their name the Hemiptera. Look for these characteristics on the
giant water bug .
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Entomology
Figure 18 The true bugs,
Hemiptera have a characteristic forewing that is
leathery at its base and
membranous at the overlapping tips. This dorsal
view makes it easy to
identify Hemiptera. ©
BIODIDAC
The Homoptera include plant sap feeding insects such as aphids, leaf
hoppers and cicadas, to name a few. Their wings differ in shape but
both have a membranous texture. The forewings are held over the
abdomen covering both the dorsal and lateral sides. In the keys this is
referred to as roof-like and shouldn't be confused with the way the
damselflies and mayflies hold their wings flat against each other and
completely above the abdomen. A cicada is available for you to look
at.
Figure 19 Homopteran
wings are both membranous but are helf like o
roof covering the dorsal
and lateral sides of the
abdomen © BIODIDAC
The greatest difference between the two pairs of wings occurs in the
beetles, Coleopetra. Here the front wing is hardened into elytra protecting the delicate hind wings underneath. The degree of hardness
varies from leathery to hard. The elytra are not involved in flight and
are lifted out of the way as the insect flies using its hindwings. Soldier
beetles and dogbane beetles are available. If during your observations
of the hind wing the forewing breaks off discard the specimen.
Not all insects have four wings, the Diptera or flies are characterised
by only two, a single pair, of visible wings. The second set is actually
still there, but reduced to halteres. Find them on the blowflies that
have been provided.
Only the Diptera have two pairs of wings but there are others that
can fool you. Many insect groups lock the two pairs of wings together
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so that they function as a single wing . A good example of this is seen
in the wasps (Hymenoptera). A series of hooks on the leading edge of
the hind wing hold it the trailing edge of the forewing . Look for the
coupling mechanism on the wasps that have been provided. Lepidoptera also couple their wings together using frenula, a long bristle that
arises at the base of the hindwing and fits into a set of hook like
scales on the forewing . Geometrid butterflies are available for you to
look at.
PAGE: 17 - EXTERNAL ANATOMY
© JON G. HOUSEMAN