CHAPTER III.
Structure of Insects.
Although insects present a great variety of forms, they nevertheless agree in general features; thus by studying the structure of some generalised species, which will give a broad idea of the main characteristics, one is enabled to recognise different structural modifications assumed by various species. For this purpose a weta, grasshopper, or cockroach may be taken as a type.
Just as in the case of the crayfish, so the body of an insect is completely covered and protected by a continuous “shell,” very solid in some insects, more or less pliable in others, but even in the most delicate forms tending to become rigid and brittle after death. This shell acts as a skeleton and as a very effective armour-plating, protecting and supporting the soft body within. Unlike the shell of the crayfish, which is mainly calcareous, that of insects consists of a horny substance called chitin, secreted by the underlying skin, and constitutes what is known as a cuticle. It is due to this horny cuticle or shell that the form and colour of most insects are preserved after death, though the enclosed body tissues decay unless preserved in some suitable medium.
The cuticle, though forming a complete covering, does not enclose the body in an inflexible shell; flexibility is allowed by the cuticle being formed of a segmented series of strongly-chitinised sections alternating with skin-like, feebly-chitinised, and very elastic sections; this arrangement gives freedom of movement to the enclosed body, as is readily seen in the movements of a caterpillar.
There are three distinctly separated divisions of the insect body—head, thorax, and abdomen—each consisting of a varying number of segments ([Fig. 3]). The head segments are so closely fused as to be practically untraceable, the cuticle forming a rigid capsule; the thorax, to which the head is attached, carries the wings (when present) and the legs, and consists of three segments; posterior to the thorax is the abdomen, comprised of several segments, which show the typical segmentation of insects better than any other part of the body.