882 



INSECTA. 



on which account it was compared by him to 

 lignin, the basis of woody fibre. He believed 

 also that it contains no carbonate of lime, the 

 earthy salts being chiefly phosphate of lime, 

 with carbonate of potass and a little phosphate 

 of iron. Mr. Children, however, by a more 

 careful and different mode of analysis, proved 

 that chitine is composed of carbon, hydrogen, 

 nitrogen, and oxygen, in about the following 

 proportions, the mean which we have deduced 

 from his details of two careful analyses : 



Grs. 



Carbon 46.08 



Hydrogen 5.96 



Nitrogen 10.29 



Oxygen 37.4 1 



99.74 



and that, in addition to the earthy salts men- 

 tioned by Odier, there are also small propor- 

 tions of silica and magnesia, and a slight trace 

 of manganese ; and it has since been stated 

 that there is likewise a trace of carbonate of 

 lime.* Some authors still imagine that chitine 

 contains no nitrogen ;f but in the careful expe- 

 riments of Mr. Children, who was assisted by 

 Professor Daniell,J the formation of prussic 

 acid, which took place during the analysis, was 

 decisive of the fact of its existence. 



Thus, then, in the distinctness of its chemical 

 composition from that of horn and other dermal 

 appendages, and in its similarity to that of true 

 bone, in the greater proportion of its earthy 

 matter being phosphate of lime, may we not 

 venture to infer that chitine, the basis of the 

 insect skeleton, is intermediate in its chemical 

 condition between the ossitic and dermal struc- 

 tures ; or, in other words, is an imperfectly de- 

 veloped condition of bony matter, so modified 

 that, while it is subservient to the great purpose 

 of animal life, in affording strength and solidity 

 to the parts in which it exists, it at the same 

 time admits of their performing all the organic 

 functions of the true skin ? 



If such be not the case, it will be difficult 

 satisfactorily to account for the solidification of 

 those internal processes which, in insects, occupy 

 the position and perform the office of the true 

 bones in vertebrata, but which are originally deri- 

 ved from the external teguments. Thus we shall 

 find that in the cranium of some of the Coleop- 

 tera, the most perfect insects, the cerebral gan- 

 glia are protected on either side by more or less 

 perfectly developed laminae of this bone-like 

 structure; that the first suboesophageal ganglion 

 actually lies in a cradle of the same, and that 

 the nervous cord itself, before passing out of 

 the cranium, is not only protected laterally by 

 continuations of these lamina?, but is often in- 

 closed in a distinct bony ring. But it may be 

 said that the exuviation of the coverings of in- 

 sects during the early period of life, when un- 



* Professor Owen's Lectures at the Royal Col- 

 lege of Surgeons, May 1837. 



t Professor Grant, Lancet, Dec. 7, 1833, p. 393. 

 Bunueistrr, Manual of Entomology, (translation,) 

 1836, p. 230. 



$ ZoolofcicalJournal, March 1824, p. 115. 



dergoing their metamorphoses, and a like con- 

 dition in other articulata, is opposed to this 

 opinion. To this we reply, that in all true in- 

 sects exuviation of the skeleton takes place 

 only during the growth and metamorphoses of 

 the individual, and that when these are com- 

 pleted, and the insect has arrived at its adult 

 condition, when its body no longer continues to 

 be enlarged, the then perfect skeleton is per- 

 sistent throughout the remainder of life, which, 

 as in the hive-bee, may continue for many 

 months, and under some circumstances, as has 

 been known among the Coleoptera, even for 

 two or three years. The exuviation of the ske- 

 leton of Crustacea, which are said to continue 

 to grow throughout the whole period of their 

 existence, is similar to that of insects, and per- 

 haps in both is induced, not alone, as usually 

 supposed, by the mere mcasement of the animal 

 in a covering which prevents the further growth 

 of its body, but by changes in the actual con- 

 dition of the skeleton itself, dependent upon 

 the same laws of existence which regulate the 

 removal of the old and the deposition of new 

 matter in the bones and other structures of the 

 vertebrata. 



Of the manner in which chitine is deposited 

 in insects we have no direct information. 

 Latreille considers it to be a solidification in the 

 mucous tissue, and Dr. Grant affirms it to be 

 a deposition upon the true skin. This appears 

 also to have been the opinion of Odier, who 

 found chitine in theexuviable skeleton of Crus- 

 tacea, in which he says it exists in the form of 

 lamellae.* In whatever form it is deposited, it 

 is intimately connected with the true corium, 

 into the composition of which it appears to 

 enter. It is covered by the colouring matter, 

 and also with a distinct epidermis like the 

 horny cuticle on the carapace of Chelonia. On 

 comparing the experiments of M. Odier and 

 Mr. Children the quantity of chitine appears to 

 vary a little in different insects.f A curious 

 circumstance mentioned also by Odier is that 

 it appears to enter into the composition of the 

 trachea? of the wings, but not into that of their 

 connecting membranes. If this be the case, it 

 is a further proof that the skeleton ought no! to 

 be compared to the epidermal appendages of 

 vertebrata. 



The skeleton consists of thirteen distinct seg- 

 ments, which are believed to be its normal 

 number in all insects. But recent observations 

 on the larva? of Hymenoptera and Diptera, 

 before alluded to, render it probable that this is 

 not the full amount, and that the number is at 

 least fourteen, at all events in some species. 

 Mr. Westwood has already shown this to be 

 the case in Hymenoptera, and that in the per- 

 fect state of Forficulal there are thirteen dis- 

 tinct segments in the male, and, also in a 

 rudimentary state, in the female, besides the 

 anal forceps. We have ourselves invariably 

 found fourteen in the apodal larvae of Hymen- 

 optera and in some of the Diptera; but we 



* Zoological Journal, vol. i. March, 1824, p. 108. 



+ Op. cit. 



t Trans. nt. Society, vol. i. p. 157, et seq. 



