INSECTA. 



881 



instances in which the life of the imago is ex- 

 tended beyond the usual period, it appears to 

 result from one of the great objects of existence 

 being unaccomplished; the insect is always in 

 a state of celibacy, in which condition the life 

 of an ephemera may be extended to several 

 days, and perhaps even to two or three weeks.* 

 1. Der mo-skeleton. The skeleton of insects 

 is formed of a modification of the external 

 coverings of the body, together with certain 

 ossified portions situated within the head and 

 thorax, to which some of the most important 

 muscles are attached. Hence it is called a 

 dermo-skeleton. The true organs of support are 

 thus placed on the exterior instead of the inte- 

 rior of the body, and the solid skeleton, impact- 

 ing the whole, as it were, in a coat of mail, gives 

 additional strength to the delicate limbs by 

 affording a larger surface for the attachment of 

 muscles, while it more securely protects the 

 bodies of these diminutive, but exquisitely 

 formed little creatures, from the injuries to 

 which they are constantly exposed. Tims, then, 

 in the strength and position of the skeleton, in- 

 sects have as striking affinities with the Chelo- 

 nian Reptiles as they have, as we shall hereafter 

 see, with Birds in the extent, distribution, and 

 activity of their respiratory organs; and with 

 the hibernating Mammalia in their maintaining 

 an elevated temperature of body only when in a 

 state of activity. Some naturalists, however, 

 have contended that the analogies which were 

 traced, first by our illustrious countryman 

 Willis in the year 1692, and subsequently 

 by Geoffroy St. Ililaire and other comparative 

 anatomists, between the dermo-skeleton of in- 

 sects and the proper skeleton of vertebrated ani- 

 mals, are incorrect, and that the structure ought 

 rather to be regarded as the analogue of the 

 skin than as that of the osseous system, and 

 hence they have compared it only with the 

 nails, horns, and other appendages of the epi- 

 dermis. These objections receive additional 

 weight and importance from the circumstance 

 that one set of organs, the elytra, which form 

 part of the hardened coverings, are actually de- 

 rived from the respiratory structures. But it may 

 be remarked in reply, that the skeleton of in- 

 sects, both in its office and ultimate composition, 

 resembles more the bones of Chelonian Reptiles, 

 which, like it, are covered with a thin cuticular 

 lamella, and placed on the exterior of the body, 

 than the true skin or the epidermis. Hence we 

 shall continue to regard and describe it as sub- 

 servient to the same purposes in these diminu- 

 tive creatures as the osseous system in vertebrata. 

 This view of its real nature is justified, as we 

 shall presently see, by analyses of its chemical 

 constituents. The peculiar characteristic of 

 bony structure is the presence of a large propor- 

 tion of a particular kind of earthy matter, and 

 this is also one of the great characteristics of the 

 coverings of insects, which become consolidated 

 during the changes, by the deposition of a quan- 

 tity of the same kind of earthy matter within 

 them. But we cannot regard the coverings thus 

 formed as merely exsiccated non-vascular struc- 



* Op. cit. p. 27. 



VOL. ir. 



tures ; on the contrary, we beliere them to be 

 nourished by the circulatory fluids, perhaps to 

 as great an extent as the external skeleton of 

 Chelonia. In support of this opinion it may be 

 remarked that those internal processes which 

 exist in the perfect state, and are developed 

 during the metamorphoses from duplicatures of 

 the external tegument, perform most important 

 offices in the body as organs of support and 

 attachment for powerful muscles. It can hardly 

 be imagined that these internal processes are 

 not nourished by the circulatory fluids like the 

 muscles that are attached to them, while it is well 

 known that every part of the external covering is 

 penetrated by ramifications of the air-vessels, the 

 course of which in the wings has recently been 

 shown to be always indicative of the passages 

 along which the blood circulates.* Hence it 

 is fair to infer that every part of the animal sup- 

 plied with tracheae is also nourished by the cir- 

 culatory fluid, as well in the exterior skeleton of 

 the thorax and abdomen as in the hardened elytra 

 and wings, in which the presence of the fluid 

 has been actually detected by its movements. 



Chemical composition. The peculiar sub- 

 stance that constitutes the hard portion of the 

 dermo-skeleton is called chitine by Odier, and 

 entomoline by Lassaigne. The most generally 

 received name is chitine. M. Odier, who first 

 analysed the coverings of insects, and disco- 

 vered this substance,f found that it constitutes 

 about one-fourth part of their whole weight, 

 and that the remaining three parts consist of 

 albumen, extractive matter soluble in water, a 

 coloured oil soluble in alcohol, and a brown 

 animal substance soluble in potass, but insolu- 

 ble in alcohol. The latter substance, which 

 exists in considerable quantity, was found by 

 Lassaigne to be analogous to the peculiar ani- 

 mal matter of cochineal, coccine, and that it 

 forms the basis of the colouring matter of the 

 skeleton. The composition of chitine has been 

 differently stated by chemists, but by all it has 

 been shown to be perfectly distinct from horn, 

 the nails, and other appendages of the epidermis, 

 in being quite insoluble in a hot solution of 

 caustic potass, and in not fusing or swelling up 

 like horn when burnt at a red heat, but leaving 

 a white ash, which retains the original form of 

 the part. This sufficiently proves that the co- 

 verings of insects cannot properly be compared 

 with the mere epidermis or its appendages. 

 According to Odier, chitine is obtained by di- 

 gesting the hard parts of the skeleton in a hot 

 solution of caustic potass, renewed several 

 times, until it has ceased to have any action 

 upon them. The solution, by removing the 

 colouring matter and other constituents, be- 

 comes of a deep brown, and leaves the chitine 

 nearly as transparent as horn, without any 

 change of form. This substance, as we have 

 before stated, constitutes about one-third or 

 fourth of the weight of the whole skeleton, and 

 was believed by Odier to contain no nitrogen, 



* Bowerbank, Observations on the circulation of 

 blood and the distribution of the trachea? in the wing 

 of Chrysopa perla, Ent. Mag. No. 17, Oct. 1836. 



t Memoires de la Societe d'Hist. Natur. de Paris, 

 torn. i. Zoological Journal, vol. i. p. 101, Mar.1824. 



3 M 



