Insects, therefore, may be characterized as a class of hexapodous invertebrate animals, which possess antenna?, and have the body com posed of several segments, united into three and sometimes four distinct parts, articulated together, consisting of head, thorax, and abdo men. They breathe atmospheric air by means of lateral spiracles and tracheae, and pass through a succession of changes of form, or shed their external covering before they arrive at their perfect state. They also possess other charac ters in common with the Myriapods and Arach nidans, as the circulation of the nutritive fluids by means of a pulsatory dorsal vessel, divided into distinct chambers or compartments, and the respiration of atmospheric air by means of spiracular orifices, and with the Crustaceans in being in general oviparous.
Anatomically considered, Insects, as re marked by Professors Grant* and Owen,f bear a remarkable analogy amongst invertebrated animals to Birds amongst the vertebrated. They constitute the most beautiful, most active, and most highly organized of any of the Inverte brated classes. Like Birds, they are inhabi tants of the air, the earth, and the waters, and the dominion of some of them is even extended to the bodies of other animals. Physiologically considered, they also resemble " the feathered tribes of air." Like them they have a more voluminous and extensive respiration, and a greater power of generating and of maintaining a higher temperature of body than any other class in the division of animals to which they respectively belong. The number of species is greater than is known in any other division of the animal kingdom, and is only exceeded, as in Fishes, by the almost countless myriads of individuals which every species produces. The metamorphoses which most of them under go before they arrive at the perfect state, and are able to fulfil all the ends of their existence, are more curious and striking than in any other class, and in the greater number of species the same individual differs so materially at its dif ferent periods of life, both in its internal as well as external conformation, in its habits, locality, and kind of food, that it becomes one of the most interesting investigations of the physiologist to ascertain the manner in which these changes are effected,—to trace the sue cessive steps by which that despised and almost unnoticed larva that but a few days before was grovelling on the earth, with its internal organi zation fitted only for the reception and assimila tion of the grossest vegetable matter, has had the whole of its external form so completely changed as now to have become an object of admiration and delight, and able to " spurn the dull earth" and wing its way into the open atmosphere, with its internal parts adapted only for the reception of the purest and most con centrated aliment, now rendered absolutely necessary for the support and renovation of its redoubled energies. But this condition of
insect life is greatly modified in the different families. Thus the most active species are diurnal insects, and are those which have the greatest development of the organs of locomo tion, accompanied, as in birds of flight, by a more voluminous respiration, and a greater force and rapidity of circulation, and consequent muscular energy and necessity for a constant supply of food, as is well exemplified in the hive-bee and its affinities. But although many species are furnished with wings for flight, these organs are not universally met with in the species of every order, neither are they con stant in the two sexes of the same species. In these instances it is always the male individual that is furnished with them. These exceptions occur among the beetles, as in the glow-worm ( Lampyris, fig. 335 & 336), in the Blattc or coek-roaches (fig. 343), in some species of of moths ( Bomitycida), and in the plant-lice (Aphides), while in other species, the ants, the individuals are furnished with wings only at a particular season of the year, and lose them immediately after the fulfilment of certain natural functions. In each of these instances, as noticed by Mr. Owen* in the ostrich and other birds unaccustomed to flight, the extent to which the respiratory organs are developed is in proportion to the habits of the species, being greatest in those of flight and least in those which reside constantly on the ground. Indeed, so varied are the forms, so diflbrent the habits and modes of life, that the division of Insects into families and tribes has afforded no small amount of difficulty to the scientific naturalist in arranging them according to their most natu ral affinities, and hence a great variety of sys tems have been proposed for this purpose, all of which perhaps are open to many objections.