We shall now refer to the functions of insects : beginning with respiration, which is the act of inhaling and exhaling the air into and out of the lungs. Quadrupeds, birds, and most of the amphibia, breathe through the mouth and nostrils. The air, when received into the lungs, is mixed with the blood, and imparts to it some thing necessary, and carries off something noxious. Some authors have asserted, that insects have no lungs ; but later ex periments and observations show, that no species wants them, or, at least, some thing similar to them ; and in many insects they are larger in proportion to their bo dies than in other animals. In most of them they lie at or near to the surface of the body, and send out lateral pores or trachea.
The respiration of insects has attracted the attention of many naturalists, and it is found that insects do not breathe through the mouth or nostrils ; that there are a number of vessels, for the reception of ,air 'placed along on each side of the body, which are commonly called spiracula, .which are subdivided into a number of smaller vessels, or bronchia ; that the vessels, or tracheae, which proceed from the pores on the sides, are not composed of a simple membrane, but are tubes formed of circular rugs ; that the spira cula are distinguishable, and are covered with a small scaly plate, with an opening in the middle like a button-hole, which is furnished with membranes, or threads, to prevent the admission of extraneous bodies.
Insects are the only animals without vertebra, in which the sexes are distin guished. Copulation is performed in them by the introduction of the parts of gener tion of the male into those of the female. All insects are either male or female, ex cept in a few of the genera of the order Hymenoptera, such as the bee, ant, &c. where individuals are to be found, which are neither male nor female ; and on that account called neuters. Among the bees, the neuters form the far greater part of the community, and perform the of labourers. Among the ants, the neuters are very numerous, and constitute the only active members of the society. It has been alleged, that these neuters are nothing but females, whose parts have not been developed for want of proper nou rishment. Oliver, however, after strict examination, is disposed to think them really different, though he does not ad duce facts sufficient to establish his opi nion. The parts which distinguish the male from the female may be divided into two classes, Tie. 1. Those which are not directly connected with generation. 2. Those which are absolutely necessary for the purposes of generation. The circum stances which have no direct communica tion with generation, which serve to point out the distinction between the sexes, are the difference of size observable in the male and female ; the brightness of the colour in each; the form and number of articulations of the antenna ; the size and form of their wings ; the presence or ab sence of a sting ; the male is always small er than the female ; the female ant is nearly six times larger than the male ; the female cochineal is from twelve to fifteen times the size of the male ; the female termes is two hundred or three hundred times the size of the male ; the colours of the male are commonly much more brilliant than those of the female ; this is particularly the case in lepidopter ous insects ; in some insects, the colour of the male is totally different from that of the female ; the antennae of the male are commonly of a different form, and larger than those of the female ; frequently the males are furnished with wings, while the females have none ; the lampyris, coccus, and blatta, and several moths, afford an example of this ; the female bee is fur nished with a sting, while the male is des
.titute of one ; the males of some insects are ft irnish ed with sharp prominent points, resembling horns, situated either on the head or breast, which are either not per ceptible, or very faintly marked, in the .female. The parts essential to generation .afford the best distinguishing mark ; in most insects they are situated near the extremity of the rectum ; by pressing the abdomen near to the anus, they may fre quently be made to protrude ; but the parts of generation are not always situat ed near the anus : in the spiders, they are . situated in the feelers ; in the libellula, .the male organ is situated . in the breast, while that of the female is placed at the ,anus.
The eggs of insects are of two sorts : the first membranaceous, like the eggs of the tortoise, and the other reptiles ; the other covered with a shell like those of the birds ; their figure varies exceed ingly ; some are round, some elliptical, some lenticular, some cylindrical, some pyramidal, some flat, some square, but the round and oval are the most common. The eggs of insects seldom increase in size, from the time they have been depo sited by the parent till they are hatched ; those of the tenthredo, however, and of some others, are observed to increase in. bulk. At first there is nothing to be per ceived in the eggs of insects but a watery fluid ; after some little time, an obscure point is observable in the centre, which, according to Swammerdam, is not the in sect itself, hut only its head, which first acquires consistence and colour : and the same author alleges, that insects do not increase in bulk in the egg, but that their parts only acquire shape and consistence. Under the shell of the egg, there is a thin and very delicate pellicle, in which the insect is enveloped, which may be com pared to the chorion and amnios, which surround the foetus in quadrupeds. The little insect remains in the egg till the fluids are dissipated, and till its limbs have acquired to break the egg, and make its escape ; the different species of insects remain inclosed in the egg forvery different periods ; some continue inclosed only a few day s, others remain for seve ral months. The eggs of many insects re main without being hatched during the whole winter, and the young insects do not come forth from them till the season at which the leaves of the vegetables, on which they feed, begin to expand. When the insects are ready to break their pri son, they commonly attempt to pierce the shell with their teeth, and form a circular bole, through which they put forth first one leg, and then another, till they ex tricate themselves entirely.