CHAPTER IV. 



THE OUTER SKELETON. 



SPECIAL REFERENCES. 



KKUKENBERG. Vergleichend-Physiologische Vortriige. IV. Vergl. Physiologic 

 <ler Thierischen Geriistsubstanzen. (1885.) [Chemical Relations of Chitin.] 



GRABEK. Ueber eine Art fibrilloiden Bindegewebes der Insectenhaut. Arch. f. 

 mikr. Anat. Bd. X. (1874.) [Minute Structure of Integument.] Also, 



VIALLANES. Recherches sur THistologie des Insectes. Ann. Sci. Nat., Zool. 

 VP Serie, Tom. XIV. (1882). 



AUDOUIN. Recherches anatomiques sur le thorax des Insectes, &c. Ann. Sci. Nat. 

 Tom. I. (1824.) [Theoretical Composition of Insect Segments.] Also, 



MILNE-EDWARDS. Lemons sur la Physiologic et TAnatomie Comparee. Tom. X. 

 (1874.) 



SAVIGNT. Memoires sur les animaux sans vertebres. Partie I c - Theorie des 

 organes de la bouche des Crustacees et des Insectes. (1816.) [Comparative 

 Anatomy of the Mouth-parts.] 



MUHR. Ueber die Mundtheile der Orthopteren. Prag. 1877. [Mouth-parts of 

 Orthoptera.] 



Cliitin. 



WHEN the skin of an Insect is boiled successively in acids, 

 alkalies, alcohol, and ether, an insoluble residue known as 

 Chitin (CisH^NsOio) is obtained. It maybe recognised and 

 sufficiently separated by its resistance to boiling liquor potassse. 

 Chitin forms less than one-half by weight of the integument, but 

 it is so coherent and uniformly distributed that when isolated 

 by chemical reagents, and even when cautiously calcined, it 

 retains its original organised form. The colour which it fre- 

 quently exhibits is not due to any essential ingredient ; it may 

 be diminished or even destroyed by various bleaching processes. 

 The colouring- matter of the chitin of the Cockroach, which is 

 amber-yellow in thin sheets and blackish-brown in dense 

 masses, is particularly stable and difficult of removal. Its com- 

 position does not appear to have been ascertained ; it is white 

 when first secreted, but darkens on exposure to air. Fresh- 



