These astonishing phenomena will perhaps be better understood if we consider the obstacles which insects have to overcome to satisfy their wants, to seek their food, to defend themselves against their enemies, &c.
To meet these requirements they are marvellously constructed for both labour and warfare, and their strength is superior to that displayed by all other animals. It is also much greater than that of the machines we construct to replace manual labour. They represent strength itself. God's workmen are infinitely more powerful than those invented by the genius of man, which we call machines.
We think it necessary, in closing this chapter, to give a sort of general outline of the great class of animals which we are about to study. If we wished to characterise insects by their exterior aspect, we might consider them as articulate animals, whose bodies, covered with tough and membranous integuments, are divided into three distinct parts: the head, provided with two antennæ, and eyes and mouth of very variable form; a trunk or thorax, composed of three segments, which has underneath it always six articulated limbs, and often above it two or four wings; and an abdomen, composed of nine segments, although some may not appear to exist at first sight.
If, in addition to these characteristics, one considers that these animals are not provided with interior skeletons--that their nervous system is formed of a double cord, swelling at intervals, and placed along the under-side of the body, with the exception of the first swellings or ganglions which are under the head--that they are not provided with a complete circulating system--that they breathe by particular organs, termed tracheæ, extending parallel to each other along each side of the body, and communicating with the exterior air by lateral openings termed spiracles--that their sexes are distinct--that they are reproduced from eggs--and, in conclusion, that the different parts we have mentioned are not complete until the creature has passed through several successive changes, called metamorphoses, a general idea may be formed of what is meant in zoology by the word "insect."
Insects, whose general organisation we have briefly traced, have been classed by naturalists as follows:--