What is an insect.—The word insect is so commonly applied to animals having no claim whatever to the title, that it is advisable to point out at once some of the features which distinguish insects from other animals with which they are often confused. Insects, spiders, crustaceans, centipedes, and their near relatives, all have jointed bodies and legs, which are covered by a continuous suit of armour of a substance called chitin. In some cases, as in lobsters and crabs, this is for the most part hardened by mineral matter to form a stout shell, being soft and flexible only at the joints where ease of movement is required. In other cases the layer of chitin may remain thin and delicate, and all gradations between the two extremes may be found. Though soft where movement of one part on another takes place, the chitin is always firm enough, elsewhere, not only to form a protection for internal organs, but also to afford attachment to the muscles which move the body and limbs. It is therefore a skeleton, but as it is on the outside of the body it is called an exoskeleton, to distinguish it from the internal skeleton (p. 224) of the vertebrata. Animals such as insects, spiders, centipedes, and crustaceans, which have jointed bodies and legs, and are covered by a chitinous exoskeleton, are called arthropods. Insects may be at once distinguished from all other arthropods by the single pair of feelers and the six legs. Many of them, but not all, possess wings—structures which are found in no other arthropods. Insects are always air-breathers when adult. The cockroach.—The cockroach is not a general favourite, but it displays so well the essential features of insect structure that it affords an excellent introduction to the study of the more popular [Pg 352] members of the class, which in many respects are highly specialised. It is also easily obtained, and of fairly large size. The body of the cockroach (Fig. 220) is very distinctly divided into three regions: (1) the head (Fig. 221) which carries the feelers, the eyes (ey.) and the jaws (man., max.¹, and max.²); (2) the thorax, separated from the head by the slender neck, and bearing the legs and the wings; and (3) the abdomen, which bears neither jaws nor limbs. Fig. 220.—Cockroach, male, seen from above. (× 1.)