Insect Structure

The imago, or ideal insect, is protected by a more-or-less flawless horny integument made of a material called chiten, which often takes the shape of a regular box holding all the internal organs. First, the head, to which are attached the mouth parts, antennae, and eyes; second, the thorax or chest, sometimes forming a solid mass, but properly composed of three segments, namely, the prothorax, mesothorax, and meta-thorax, to which are attached three pairs of legs and two pairs of wings – there are, however, many exceptions to the last, as some have only one pair, and others are wingless.

It seems very difficult to account for the countless millions of some of the smaller insects, such as aphids and scale insects, which suddenly swarm as if by magic over plants. Again, despite what many of our farmers may sincerely claim, a grain weevil cannot transform into a flour moth or vice versa. The moment the tiny caterpillar or grub emerges, it immediately begins to eat. It spends the entirety of this stage of development eating and growing, periodically moulting by shedding its outer skin to accommodate its expanding bulk. When it is fully grown and prepared for its final moult, it stops eating, crawls away into a suitable location, and creates a cocoon, cell, or shell in which it pupates. It is now a chrysalis or pupa, immobile save for a small twitch of the abdomen tip when disturbed. The various organs of the ideal insect gradually take shape beneath the pupal covering until the final stage of evolution is reached one sunny day. The flawless insect emerges from its swaddling cloths with a few convulsive spasms, revealing itself in all of its beauty and perfection.

Some groups, however, go through a much simpler or less complete metamorphosis, emerging from the egg as a baby insect ready to eat (like the grasshopper), the same food as its mother; going through a series of moults, and after casting- off its skin each time, becoming more nearly perfect without any true pupal stage, and finally, after the last moult, coming forth with well-developed wings, a perfect insect.

The typical insect has two or three simple eyes that form brightly shining dots between the compound pair, known as ocelli, and a large compound eye on either side of the head made up of a number of small sections called facets, ranging in number from sixteen to several thousands in some of the more highly developed families. Some populations lack these ocelli, while others lack both eyes and ocelli, rendering the insects blind.

The mouth parts are made up of many hard plates; in chewing or biting (mandibulate) insects, they consist of a pair of strong jaws with the labrum in front of them and the maxillae behind them, followed by a second set of jaws that have been fused together to create the labium. The labium and maxillae are both equipped with a pair of slender jointed appendages called labial and maxillary palpi, which are utilised as fingers to help bring food into the mouth. As an insect is feeding, its numerous sections combine into a simple sucking tube with a pointed style-like tip that is buried in the plant tissue. This process occurs in groups with sucking (haustellate) mouths. When fully developed, the antennae are made up of several different rings or segments that protrude from either side of the head and are often attached to the front of the head between the eyes. These organs are used for touch, smell, and possibly hearing.

The coxae, or hips, are the first of five separate joints in the legs. The trochanters, which are little joints with a ball and socket motion from which the femora, or thighs, move backwards and forwards, are attached to these, and the tibiae, or shanks, end in the tarsi, or feet, at the extremity. The majority of insects also have two tarsal claws, sometimes with a tiny pad called the pulvillus or empodium between them.

Insect wings come in a wide variety. The fore pair of many insects are solid chitinous wing cases that serve primarily as protective covers for the hind pair, which, while the insect is at rest, lay folded up beneath them. Some are membranous and smooth, while others are covered in down or scales. The nervures, branching tubes that cross the flying wings and serve to reinforce them as well as play a crucial role in the insect’s breathing, are punctured with tiny apertures that are particularly noticeable in some hymenoptera species.

Insects breathe through openings called spiracles that are located along the sides of the thoracic and abdominal segments. These openings branch out into air vessels called trachea, which pass into the interior of the body before ramifying into every part and appendage, including the tips of the antennae. A double chain of ganglia makes up the nervous system, which controls the insect’s life and movement.

Nerve cells that are attached to smaller nerve tissues that encircle them and extend outward before returning to the ganglia, which control nerve sensation. They cannot feel to the same degree as vertebrate creatures since their perceptions are so much less restricted to the brain. As a result, it’s common to see a locust, beetle, or ant that has escaped from a bird still crawling around, perfectly capable of moving all of its remaining organs. A dragonfly can even have its long, skinny body removed. Carefully insert a piece of grass stalk of the same length and weight to balance the wings, and it will fly off pretty easily. Of course, they won’t survive long after such wounds.

Since many insects would descend to the ground at the slightest alarm even before the bush they are resting on has been touched, we know that many of them must have extremely keen hearing or air movement perceptions. Except for grasshoppers and locusts, which have ears that are placed at the base of the abdomen or on the front leg, and some species of which may be seen with a regular lens, very little is known about the hearing organs.

Insects typically only have two sexes, male and female; but, among those that live in social colonies, such as bees, ants, wasps, and termites, the bulk of the inhabitants are neuters. These neuters, who are typically aborted females, carry out all of the nest-building effort and maintain the community’s food supply.