The picture above is the first of a series of computer models that BotRejectsInc is developing to illustrate insect
anatomy and structure. Only the basal parts of the six legs have been added so far (for clarity) and head and
tail appendages are also omitted. Bot intends to section this model and show the main internal organs. The
salient external features of a 'typical' insect, like the one above, are discussed below.

Introduction

Have you ever pondered what remarkable machines insects are? They are chemical and mechanical machines
engineered to incredibly high standards by mother Nature. The exoskeleton ('shell') contains a material called
chitin, which is as hard and tough as copper metal. The only reason you can easily squash an insect is
because you are like the Empire State building compared to it! If it were as big as you, then an armour-piercing
anti-tank round might come in handy. Inside its head is a tiny but extremely sophisticated computer that far
out-performs any made by human hands. Think about the amazing aerobatic displays that dragonflies put on.
When a dragonfly intercepts prey on the wing, its brain has to receive information from a multitude of sensors,
then compute all the vectors and calculate the correct intercept trajectory and then inform the various moving
parts what to do - all in an instant! I have yet to see any engineer on Earth program a robot that can do this
and everything else the dragonfly must do! Even if they did, its brain would have to be so big that it could not fly
with such small wings! What is more, dragonflies can hover and fly backwards if they want to and they can
direction in mid-air and can reach speeds of over 35 mph. Not bad for such small motors! Dragonflies use
motion camouflage to surprise one another during air-space disputes, and probably also use it to surprise
their prey - they keep themselves between the target and a distant landmark, which makes it hard to see that
they are rapidly approaching but makes them appear stationary instead! This is rather like Sir Lancelot in
Monty Python's Holy Grail! Dragonfly adults lay their eggs in ponds and can often be seen near ponds in
woodlands or near canals. This one was found near the Birmingham-Worcester canal, I once saw a large gold
and black species there that was very magnificent, but I never had my camera handy at the time!

Indeed, the brain is not the only computer inside the dragonfly. Every pair of legs has its own local processor
and, as we shall soon see, every cell is in fact a computer. Thus, think what wonderful machines insects are
before you squash them. They make a Rolls Royce look like primitive trash, but one wouldn't go around
squashing Rolls Royces!

Insect Anatomy

Insects are encased in a tough exoskeleton of armour plates which also serve to support the muscles which
attach to the inside of the exoskeleton. This exoskeleton is also called the cuticle and is made up mainly of a
polymer called
chitin. Where elasticity is needed, such as at the joints, the cuticle also contains large amounts
of a protein called
resilin. Resilin is extremely elastic and rubbery and is a component of the articular
membranes - those flexible regions of the cuticle that span the joints. Other regions of the cuticle are more or
less elastic, the strong legs of grasshoppers, for example, have some elasticity in their cuticle, which stores the
energy of the contracting muscles like springs and release this energy when the grasshopper jumps. Your
human body uses similar principles - about one third of the 'power' that you use when you sprint or jump comes
from the elastic recoil of your bones, ligaments, tendons and joints.

Insects belong to a very large and extremely diverse group of animals, called the
arthropods. The name
'arthropod' literally means 'jointed leg' as arthropods all have tough exoskeletons that use jointed limbs for
movement. Examples include crustaceans like crabs, lobsters, shrimps and woodlice; spiders and scorpions
and similar creatures; millipedes and centipedes; and insects. Insects have six legs (three pairs) and so are
sometimes called the hexapods. Most insects also possess one or two pairs of wings. Some insects are
flightless, either because they or certain members of their society lost their wings in the cause of evolution (e.g.
worker ants are wingless, but new kings and queens are winged, fleas and lice have lost their wings completely)
or because they never evolved them (presumably the first insects were wingless).

The body of the insect is divided into three main regions:
the head, the thorax and the abdomen. The
thorax consists of three body segments, each possessing one pair of legs. Early fossil insects may possess
three pairs of wings, one on each thoracic segment, and these wings appeared to have evolved from flanges,
folds, evaginations or outgrowths of the cuticle (insect wings consist of two sheets of cuticle with supporting
veins). However, even in these forms the first pair of wings is typically reduced and in modern insects only the
second and third thoracic segments may bear wings. In many insects the wings on the third thoracic segment -
the hind-wings, are also absent. Honey bees beat their wings at around 200 times per second, whilst large
butterflies may beat their wings less than 10 times per second, mosquitoes at 400-600 times per second and
very small insects, like
Forcipomyia (midge) at 2218 times per second! In flies the hind-wings have been
reduced to sensory structures called halteres, so flies have two wings only.
Insects
The body of the insect is made up of a series of segments. The insect above has three obvious thoracic
segments and nine abdominal segments, but there are actually more than 12 segments present, as we shall
see. Each segment was originally more similar to the other segments, but over the course of evolution the
segments grouped into the head, thorax and abdomen and each began to differ from the others as a specific
function was assigned to each segment. To see how this scheme of evolution / development works
click here
and read about animal bodies. Each segment originally had its own 'brain' effectively, or rather a pair of
brains, since most structures were paired. Each of these local brains or
ganglia (as they are more properly
called, sing. ganglion) would have coordinated the appendages attached to that segment and the various
sensors. Each segment probably had its own pair of appendages, controlled by its own pair of ganglia and so
was pretty much independent. This conditions still exists in many worms, in which the head is not so well
developed (indeed many worms can grow a new head should they lose one!). The head of the insect evolved
as several anterior segments fused together. The insect brain proper, consists of a series of paired ganglia
fused together, suggesting that the head developed from the fusion of 4-6 segments or so. In many insects,
the three thoracic segments may be more-or-less segments, and they may appear to be a single long
segment, especially when viewed from above. The fusing of neighbouring segments into groups enables
them to better perform specific functions. The proximity of the various ganglia in the brain facilitates
computation since it is easier for them to share information quickly.

The original limbs of the head segments have been modified into mouthparts and 'feelers'. (See
insect
nutrition for details). The mouthparts of insects are extremely diverse, as some insects crush their food, some
suck through a needle-like proboscis, etc. Often there is a pair of visible
mandibles, that bite and crush
(sometimes called 'pincers' or jaws). These are obvious in ants and can give painful bites (though many ants
also have some sort of sting in their tails). Notice that in insects the mandibles bite tranversely (horizontally)
whereas in mammals the jaws bite vertically. There are also little arm-like segmented appendages, called
palps, of which there are typically two pairs. These palps are sophisticated sense organs that probe and
examine food. In some insects the palps are so large as to resemble a 4th pair of legs and they may palpitate
or probe the floor as the insect walks. This reminds us that the palps are modified legs. There is also a
pair
of chitinous lips
(the labrum is the top lip, and the labium is the bottom lip). Just beneath/behind the
mandibles is a pair of moveable jaw-like appendages called the
maxillae. The maxillae handle the food and
guide it to the mouth, after the mandibles have crushed and pulped it. The mouth is located at the base of
the mandibles, which flank it on either side (note a model of a 'typical' insect's mouthparts will shortly appear
to help clarify these parts).

Mounted on the top and front of the insect head, between the main pair of eyes, are the 'feelers' which are
more properly called antennae, since they do more than simply feel, in fact they are extremely sophisticated
sense organs. The antennae and palps are segmented, with a series of joints, and retain an obvious leg-like
appearance, though usually much modified.

Also on the head are numerous light sensors. In most insects, most of these are grouped into two large
multi-faceted and multi-lensed arrays, called the
compound eyes. However, on top of the head, between
the compound eyes, there is usually a group of three or so much smaller and simpler eyes, called
ocelli. The
ocelli are poor at perceiving images, but are very sensitive to small changes in light intensity and may help
the insect keep track of the time of day. The compound eyes, on the other hand, are the main image-forming
eyes of the insect, with which it can discern objects and patterns, and possibly colours. If you look closely at a
housefly, you should be able to make out the three ocelli as well as the two obvious compound eyes, but a
magnifying lens would be helpful.

Each segment of the insect body is encased in a series of cuticular plates. In the thoracic segments there are
four principal plates - a topmost
tergite (or tergum, also called the notum in the thorax) which is often the
most heavily armoured, a bottom-most
sternite (or sternum) and a pair of side-plates called pleura or
pleurons (singular pleuron) or pleurites. [Although often used interchangeable, the pleurites, tergites and
sternites refer specifically to chitinous plates that form part of the pleura, tergum and sternum.] The term
sclerite refers in general to any chitinous plate in the exokeleton.

The pleurons are usually the least heavily armoured and may be more flexible that the sternite and tergite. In
the abdomen there are no pleurites as such, just the flexible
pleural membrane that joins the sternite to the
tergite. Mounted on the side of each abdominal segment (actually on the tergite or the pleural membrane
(pleuron) or sometimes on the sternite) there may be a small opening called a
spiracle. At most there are
ten pairs of spiracles, one on each of the second and third thoracic segments, and one on each of the first
eight abdominal segments (though there may be fewer). Note that I am numbering the segments from the
front end of the animal, backwards. These open into tubes that carry air - they are breathing pores and can
often be opened or closed (they may close to reduce water loss). These tubes branch extensively, forming
internal tree-like structures running down each side of the insect body, either side of the gut, which, if you
have ever opened up an insect, have a silvery sheen where they contain air. These tubes, called
tracheae,
carry oxygen to the tissues of the insect, and carbon dioxide from the tissues to the outside. Insects do not
have lungs! (Though some aquatic insects have gills). If you look at a resting wasp or bee, you will notice that
the abdomen pulsates, these are breathing movements, that help pump oxygen in and out of the trachea
(and help to pump blood around the body) but not all insects exhibit such breathing movements.
The diagram above shows the rove beetle, Aleochara bilineata in side view. Note the typical insect features
- three thoracic segments, each bearing one pair of legs, the antennae on the head (modified legs) and the
many abdominal segments. It is usually hard to observe the body segments of an adult beetle when looking
down from above, since much of the body is covered by the pair of
elytra or wing covers (actually modified
fore-wings), but in rove beetles the elytra (born on the top of the second thoracic segment) are short, freeing
the tail which may arch over the head of the animal when it is threatened (as in the devil's coach-horse
beetle). Glands in the tail may release noxious chemicals to put other animals off eating the beetle. The
single pair of wings are not visible since they are folded beneath the short elytra, but when unfolded are
about as long a the abdomen. This neat packaging away of the wings when they are not in use protects
them when the animal is burrowing, catching prey or fighting. Note also the heavy tergal shield on the top of
the first thoracic segments, just in front of the elytra and behind the head. Such a shield or
scutellum is
typical of many insects and is probably protective.
Aleochara will only fly in direct sunlight and direct
exposure to sunlight will almost immediately cause these insects to take to the air - many beetles are
nocturnal, but may fly in twilight.

Having looked briefly at insect anatomy, the following sections will link to specific insect body systems and
aspects of insect biology:
3D insect model
3D insect model labeled
The thorax and insect locomotion
The Insect Abdomen and Reproduction
The ant is a good insect to study: it's external anatomy can be clearly
seen and it has very sophisticated social behaviour. The three segments
of the thorax can be clearly seen: the first segment or prothorax, the
second segment or mesothorax and the third segment or metathorax.
Three ocelli or simple eyes accompany the two compound eyes. The
mandibles, however, play little direct role in feeding as ants have sucking
rather than biting mouthparts, but the prominent mandibles are used in
nest construction work, to lift and carry objects and in defense. Ants are
atypical in that most never develop wings, only the males (kings) and
fertile females or queens develop wings. This loss of flight in most of the
workers is a secondary adaptation, as ants evolved from flying insects.