854 



INSECTA. 



spheric air by means of ramified tracheae from 

 Arachnida in the body being constantly divided 

 into a distinct head, thorax, and abdomen 

 and from Myriapoda, in the body being com- 

 posed in general of thirteen segments. 



Insects, therefore, may be characterized as 

 a class of hexapodous invertebrate animals, 

 which possess antennae, 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 suc- 

 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 



* Lectures on Comp. Anatomy, Lancet. 1833-34. 

 , __ ' p. 246. ' 



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, Jig. 335 & 336), in the Blatta 

 or cock-roaches (jig. 343), in some species of 

 of moths (Bombycidf), 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 different 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. 

 But it is not in the mere division of Insects 

 into families and tribes that the philosophic 

 naturalist meets with the greatest difficulty, but 

 in assigning the situation which the whole 

 class ought to occupy in the animal kingdom, 

 both in regard to Insects themselves, and in 

 their relations to other animals. Whether 

 naturalists adopt as the basis of arrangement 

 the development and perfection of the nervous 

 system or that of the skeleton, with the organs 

 of circulation and digestion, as compared with 

 similar parts in other classes, they have usually 

 been led to admit that while Insects are superior 

 to many groups, which have been placed above 

 them, in the former respects, they are inferior to 

 them in the latter; and hence, although that 

 portion of the animal body which is so all- 

 important to active existence, the nervous 

 system, is employed without hesitation as the 

 fundamental type and principle of arrangement, 

 and in the vertebrated classes is scarcely ever 

 departed from, it has become in the hands of 

 many naturalists only of secondary importance 

 in the invertebrated, and the greater perfection 



* See AVES, vol. i. p. 341. 



