The internal parts of insects are less perfect and distinct than those of larger animals, and of course less known. The brain of insects is altogether different from the substance which bears that name in other animals, being little more than ganglions of nerves, two in number, that are observed in the crab, lobster, die. The muscles consist of fasciculi of fibres, that serve apparently the office of producing two motions, namely, that of ex tending and that of bending. Some insects appear to be furnished with some floating vessels, which secrete a fluid varying co lour in different tribes, but very similar to saliva. The (esophagus, or organ of degluti tion, is a straight short tube, consisting of an nular muscular fibres, like the proboscis of the common fly.
The organs of digestion vary very much in different tribes of insects. 11fost have a single stomach, but some have it double, and others have a manifold stomach. In bees the stomach is membranaceous, fitted to receive the nectar of flowers : the bug, the boat fly, and such as feed on animal substances, have a muscular stomach. The beetle, ladybird, earwig, and some others that feed on other insects, have a double stomach, the first of which is muscular, after the manner of a giz zard, and the second is a membranaceous canal. Insects such as the cricket and grass hopper, which have many stomachs, seem to employ them much after the manner of the ruminating animals.
Instead of organs of respiration, it has been found that they have spiracula running on each side the body that serve for the reception of the air, and other vessels proceeding from these that serve for the expiration of air. Insects, among the ancients, were reckoned to be bloodless animals ; but it has since been ascertained that the process both of circulation and secretion goes forward in the bodies of insects, although in a different manner. The process of secretion is supposed to be perform ed by means of a number of long slender ves sels, which float in the internal cavity of the body, serving to secrete different fluids, ac cording to the nature of the animal ; thus the bee, wasp, sphinx, dm- have two vessels situated at the bottom of the sting, through which they discharge an acrid fluid. From the ant is extracted an acid well known to chymista, and other insects have other fluids peculiar to themselves. As to the process of circulation in insects, little more is known at present than that a contraction and dilatation of the vessels is observable in some kinds, par ticularly in caterpillars ; but the fluid which is supposed to supply the place of blood is not of the same colour.
The sexes in insects are of three kinds, namely, the males, the females, and the neu ters, which have not the usual marks of either sex. The sexes are distinguished by the dif ference of size, brightness of colours, form of the antenna, &c. ; the male is always smaller than the female, and in some cases the female is several hundred times bigger than the male; on the other hand, the males have brighter colours and larger antenna. In many cases the females have no wings ; and in some in stances, as that of the bee, the female has a sting, but the male none.
The metamorphoses of insects is one cha racteristic of these animals which distin guishes them from all others. In most insects the egg is the first state ; but there are exam ples of viviparous insects, as in the case of the aphis, the fly, &c. The insect in the second or caterpillar state, is now called the larva, but formerly the eruca. The larva differ very much in different insects ; those of the butter fly and moth are properly called caterpillars, those of the flies and bees are called maggots. The larva of the beetle tribe differ from the complete insect only by being destitute of wings. Butterflies, m their caterpillar state, are very voracious, but in their complete state they are satisfied with the lightest and most delicate nutriment. The third state into which insects transform themselves is the pupa, or chrysalis. In most of the beetle tribe the pupa is furnished with short legs, but the pupa of the butterfly tribe is without legs : that of the fly tribe is oval, but that of the bee tribe is very shapeless. The last and perfect state of insects is called by Linnaeus the imago, in which state it continues until its extinction. The life of insects varies as to its duration. Some, as bees and spiders, are supposed to live for a considerable time ; but others will not live beyond a year, a day, or some hours, in their perfect state, although they will con tinue for some time in their larva state. Water insects generally live longer than land insects. As to the classification of insects, it suffices here to observe, that Lirmaus, whose system is now generally followed, has classed them according to their wings into seven orders, namely, 1. Coleoptera, or such as have shells that cover the wings, as the beetle tribe.