INSECTS, in natural history. We have, under the article ENTOMOLOGY, given an account of the Linnzean system of this department of natural history. We shall, in this place, enumerate some of those circumstances which form the line of distinction between insects and other animals. Insects are not furnish ed with red blood, but instead of it their vessels contain a transparent lymph. This may serve to distinguish them from the superior animals, but it is common to them with many of the inferior ; though Cuvier has lately demonstrated the exist ence of a kind of red blood in some of the vermes. They are destitute of inter nal bones ; but, in place of them, are fur nished with a hard external covering, to which the muscles are attached, which serves them both for skin and bones ; they are likewise without a spineformed of vertebra; which is found in all the su perior classes of animals. They are fur nished with articulated legs, six or more ; this circumstance distinguishes them from all other animals destitute of a spine formed of vertebra. A very great number of insects undergo a metamorphosis : this takes place in all -the • winged insects. They frequently change their skin in the progress of their growth. A very great number of insects are furnished with jaws placed transversely.
The wings with which a very great number of insects are furnished distin guish them from all other animals, which are not furnished with a spine compos ed of vertebrae. Insects are generally oviparous ; scorpions and aphides, dur ing the summer months, are viviparous. Insects have no nostrils, are destitute of voice : they are not furnished with a distinct heart, composed of ventricle and auricle. Incubation is not necessary for hatching their eggs. Insects, like all other organized bodies, which form the animal and vegetable kingdoms, are com posed of fluids and solids. In the four superior classes of animals, viz. quadru peds, birds, reptiles and fishes, the bones form the most solid part, and occupy the interior part both of the trunk and limbs ; they are surrounded with muscles, liga ments, cellular membrane, and skin. The matter is reversed in the class of insects ; the exterior part is most solid, serving at the same time both for skin and bones ; it encloses the muscles and internal organs, gives firmness to the whole body, and, by means of its articu lations, the limbs, and different parts of the body, perform their various motions. In many insects, such as the crab, lob ster, &c., the external covering is very hard, and destitute of organization ; it is composed of a calcareous earth, mixed with a small quantity of gelatine, formed by an exudation from the surface of the body. As its great hardness would check the growth of the animal, nature has pro vided a remedy ; all of these crustaceous insects cast their shell annually. See Causrs. The skin of most of the other insects is softer and organized, being formed of a number of thin membranes, adhering closely to one another, and put ting on the appearance of horn. It owes its greater softness to a larger proportion of gelatine. The muscles of insects con sist of fibres formed of fasciculi ; there are commonly but two muscles to pro duce motion in any of their limbs, the one an extensor, the other a flexor. These muscles are commonly attached to a ten don, composed of a horny substance, con nected to the part which they are des tined to put in motion. In most insects, the brain is situated a little above the esophagus; it divides into two large branches, which surround the esopha gus, and unite again under it, from which junction a whitish nervous cord pro ceeds, corresponding to the spinal mar row of the superior animals, which ex tends the whole length of the body, forming in its course twelve or thirteen knots or ganglions, from each of which small nerves proceed to different parts of the body. Whether insects be en dowed with any senses different from those of the superior animals cannot ea sily be ascertained. It appears pretty evident that they possess vision, hearing, smell, and touch ; as to the sense of taste, we are left to conjecture, for we are ac quainted with no facts, by which we can prove that insects do or do not enjoy the sense of taste. The eyes of insects are of two kinds; the one compound, corn. posed of lenses, large, and only two in number ; the other are small, smooth, and vary in number, from two to eight. The small lenses, which form the com pound eyes, are very numerous ; they amount, in some insects, to many hun dreds. The far greater number of insects
have only two eyes, but some have three, as the scolopendra ; some four, as gyrinus; some six, as scorpions ; some eight, as spiders. The eyes of insects are com monly immoveable ; crabs, however, have the power of moving their eyes. That insects are endowed with the sense of hearing can no longer be disputed, since frog-hoppers, crickets, &c. furnish us with undeniable proofs of the fact. Nature has provided the males of these insects with the means of calling their females, by an instrument fitted to produce a sound which is heard by the latter. The male and female death-watch give notice of each other's presence, by repeatedly striking with their mandibles against old wood, &c. their favourite haunts. Their ears have been discovered to be placed at the root of their antenna, and can be distinctly seen in some of the larger kinds, as the lobster. That insects enjoy the faculty of smelling is very evident; it is the most perfect of all their senses.— Beetles, of various sorts, the different spe cies of dermestes, flies, &c. perceive at a considerable distance the smell of ordure and dead bodies, and resort in swarms to the situations in which they occur, either for the purpose of procuring food, or lay ing their eggs. Insects feed on a great variety of substances; there are few things either in the vegetable or animal king doms which are not consumed by some of them. The leaves, flowers, fruit, and even the ligneous parts of vegetables, af ford nourishment to a very numerous class; animal bodies, both dead and alive, even man himself, is preyed on by many of them : several species of the louse, of the acarus, of the gnat, and the common flea, draw their nourishment from the sur face of his body ; the pules ulcerans pe netrates the cuticle, and even enters his flesh. A species of gadfly, oestrus hominis, deposits its eggs under his skin, where the larva feed. Other caterpillars insin uate themselves into different cavities of his body. All the inferior animals have their peculiar parasitical insects, which feed on them during their life. There are some insects which can feed only on one species. Many caterpillars, both of moths and butterflies, feed on the leaves of some particular vegetable, and would die, could they not obtain this. There are others which can make use of two or three kinds of vegetables, but which never attain full perfection, except when they are fed on one particular kind ; for ex ample, the common silk-worm, which eats readily all the species of mulberry, and even common lettuce, neither attains so great a size, nor produces so much silk, as when fed on the white mulberry.— There are a great many which feed indis criminately on a variety of vegetables. Almost all herbivorous insects eat a great deal, and very frequently ; and r ost of them perish, if deprived of food but for a short time. Carnivorous insects can live a long while without food, as the carabus, ditiscus, &c. As many insects cannot trans port themselves easily in quest of food, to places at a distance from one another, na ture has furnished the perfect insects of many species with an instinct, which leads them to deposit their eggs in situations where the larva, as soon as hatched, may find that kind of food which is best adapt. ed to their nature. Most of the butter flies, though they flutter about, and col lect the nectareous juice of a variety of flowers, as food for themselves, always de. posit their eggs on or near to those vege tables destined by nature to become the food of their The various species of ichneumon deposit their eggs in the bodies of those insects on which their lar va feed. See lcmextnimx. The sirex and sphex are likewise careful to deposit their eggs in situations where their larva, when hatched, may find subsistence. The sphex figulus deposits its eggs on the bodies of spiders which it has killed, and encloses it in a cell composed of clay. Some in sects, at different periods of their exist ence, makes use of aliment of very differ ent properties ; the larva of some are car nivorous, while the perfect insect feeds on the nectareous juice of flowers : e. g. sires, ichneumon, &c. The larva of most of the lepidopterous insects feed on the leaves and young shoots of vegetables, while the perfect insects either take no food at all, or subsist on the sweet juice which they extract from flowers: indeed, the construction of their mouths pre yenta them from taking any other than fluid food.