_What is an Insect?_ When we remember that the insects alone comprise four-fifths of the animal kingdom, and that there are upwards of 200,000 living species, it would seem a hopeless task to define what an insect is. But a common plan pervades the structure of them all. The bodies of all insects consist of a succession of rings, or segments, more or less hardened by the deposition of a chemical substance called chitine; these rings are arranged in three groups: the head, the thorax, or middle body, and the abdomen or hind body. In the six-footed insects, such as the bee, moth, beetle or dragon fly, four of these rings unite early in embryonic life to form the head; the thorax consists of three, as may be readily seen on slight examination, and the abdomen is composed either of ten or eleven rings. The body, then, seems divided or _insected_ into three regions, whence the name _insect_. The head is furnished with a pair of antennae, a pair of jaws (mandibles), and two pairs of maxillae, the second and basal pair being united at their base to form the so-called labium, or under lip. These four pairs of appendages represent the four rings of the head, to which they are appended in the order stated above. A pair of legs is appended to each of the three rings of the thorax; while the first and second rings each usually carry a pair of wings. The abdomen contains the ovipositor; sometimes, as in the bees and wasps, forming a sting. In the spiders (Fig. 1), however, there are no antennae, and the second maxillae, or labium, is wanting. Moreover, there are four pairs of legs. The centipedes (Fig. 2, a Myriopod) also differ from the rest of the insects in having an indefinite number of abdominal rings, each bearing a pair of legs. [Illustration: 1. Spider (Tegenaria).] On examining the arrangement of the parts within, we find the nervous cord, consisting of two chains of swellings, or nerve-knots, resting upon the floor or under side of the body; and the heart, or dorsal vessel, situated just under the skin of the back; and in looking at living caterpillars, such as the cut-worm, and many thin-skinned aquatic larvae, we can see this long tubular heart pulsating about as often as our own heart, and when the insect is held against its will, or is agitated, the rapidity of the pulsations increases just as with us. [Illustration: 2. Centipede.] Insects do not breathe as in the higher animals by taking the air into the mouth and filling the lungs, but there are a series of holes or pores along the side of the body, as seen in the grub of the humble bee, through which the air enters and is conveyed to every part of the body by an immense number of air tubes. (Fig. 3, air tubes, or tracheae, in the caudal appendage of the larva of a dragon fly). These air tubes are everywhere bathed by the blood, by which the latter becomes oxygenated. [Illustration: 3. Caudal appendage of larva of Agrion.]