The Structure of Insects

T HE insect body is made up of ring-like segments which are grown together. These segments are divided into groups according to their use and the organs which they bear. Thus the segments of an insect's body are grouped into three regions, the head, the thorax and the abdomen. The head bears the eyes, the antennæ, and the mouth-parts. On each side of the head of the adult insect may be seen the compound eyes; these are so called, because they are made up of many small eyes set together, much like the cells of the honeycomb. These compound eyes are not found in larvæ. In addition to the compound eyes, many adult insects possess simple eyes; these are placed between the compound eyes and are usually three in number. Often they cannot be seen without the aid of a lens.

A part of the compound eye of an insect, enlarged.

The antennæ or feelers are composed of many segments and are inserted in front of the eyes or between them. They vary greatly in form. In some insects they are mere threads; in others, like the silk-worm moths, they are large, feather-like organs.

Grasshopper, with the parts of the external anatomy named.

The mouth-parts of insects vary greatly in structure and in form, being adapted to the life of the insect species to which they minister. Some insects have jaws fitted for seizing their prey, others for chewing leaves, others have a sucking tube for getting the juices from plants or the blood from animals, and others long delicate tubes for sipping the nectar from flowers.

The mouth-parts of a grasshopper dissected off, enlarged and named.

In the biting insects, the mouth-parts consist of an upper lip, the labrum, and under lip, the labium, and two pairs of jaws between them. The upper pair of jaws is called the mandibles and the lower pair, the maxillæ (sing. maxilla).  There may be also within the mouth, one or two tongue-like organs. Upon the maxillæ and upon the lower lip there may also be feelers which are called palpi (sing. palpus). The jaws of insects, when working, do not move up and down, as do ours, but move sidewise like shears. In many of the insects, the children are able to observe the mandibles and the palpi without the aid of a lens.

A sphinx moth with the sucking tongue unrolled.  Photo by M. V. Slingerland.

The thorax is the middle region of the insect body. It is composed of three of the body segments more or less firmly joined together. The segment next the head is called the prothorax, the middle one, the mesothorax, and the hind one, the metathorax. Each of these segments bears a pair of legs and, in the winged insects, the second and third segments bear the wings. Each leg consists of two small segments next to the body, next to them a longer segment, called the femur, beyond this a segment called the tibia, and beyond this the tarsus or foot. The tarsus is made up of a number of segments, varying from one to six, the most common number being five. The last segment of the tarsus usually bears one or two claws.

A tree-hopper, showing the mouth as a long, three-jointed sucking tube, at a.

While we have little to do with the internal anatomy of insects in elementary nature-study, the children should be taught something of the way that insects breathe. The child naturally believes that the insect, like himself, breathes through the mouth, while as a matter of fact, insects breathe through their sides. If we examine almost any insect carefully, we can find along the sides of the body a series of openings. These are called the spiracles, and through them the air passes into the insect's body. The number of spiracles varies greatly in different insects. There is, however, never more than one pair on a single segment of the body, and they do not occur on the head. The spiracles, or breathing pores, lead into a system of air tubes which are called tracheæ (tra'-ke-ee), which permeate the insect's body and thus carry the air to every smallest part of its anatomy. The blood of the insect bathes these thin-walled air tubes and thus becomes purified, just as our blood becomes purified by bathing the air tubes of our lungs. Thus, although the insects do not have localized breathing organs, like our lungs, they have, if the expression may be permitted, lungs in every part of their little bodies.

The sphinx caterpillar, with the parts of the external anatomy named.