AUDOUIN. Recherches anatomiques sur le thorax des Insectes, &c. Ann. Sci. Nat. Tom. I. (1824.) [Theoretical Composition of Insect Segments.] Also, MILNE-EDWARDS. Leçons sur la Physiologie et l’Anatomie Comparée. Tom. X. (1874.) SAVIGNY. Mémoires sur les animaux sans vertèbres. Partie I^{e.} Théorie des organes de la bouche des Crustacées et des Insectes. (1816.) [Comparative Anatomy of the Mouth-parts.] MUHR. Ueber die Mundtheile der Orthopteren. Prag. 1877. [Mouth-parts of Orthoptera.] _Chitin._ When the skin of an Insect is boiled successively in acids, alkalies, alcohol, and ether, an insoluble residue known as Chitin (C_{15}H_{26}N_{2}O_{10}) is obtained. It may be recognised and sufficiently separated by its resistance to boiling liquor potassæ. 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 frequently 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 composition does not appear to have been ascertained; it is white when first secreted, but darkens on exposure to air. Fresh-moulted Cockroaches are white, but gradually darken in three or four hours. Lowne[30] observes that in the Blow-fly the pigment is “first to be met with in the fat-bodies of the larvæ. These are perfectly white, but when cut from the larva, and exposed to the air, they rapidly assume an inky blackness.... When the perfect insect emerges from the pupa, and respiration again commences, the integument is nearly white, or a faint ashy colour prevails. This soon gives place to the characteristic blue or violet tint, first immediately around those portions most largely supplied with air vessels.” Professor Moseley[31] tells us that, thinking it just within the limits of possibility that the brown coloration of the Cockroach might be due to the presence of silver, he analysed one pound weight of Blatta. He found no silver, but plenty of iron, and a remarkable quantity of manganese. That light has some action upon the colouring matter seems to be indicated by the fact that in a newly-moulted Cockroach the dorsal surface darkens first. [30] Anatomy of the Blow-fly, p. 11. [31] Q. J. Micr. Sci., 1871, p. 394. Chitin is not peculiar to Insects, nor even to Arthropoda. The pen of cuttle-fishes and the shell of Lingula contain the same substance,[32] which is also proved, or suspected, to occur in many other animals. [32] Krukenberg. Vergl. Physiologische Vorträge, p. 200. Halliburton, Q. J. Micr. Sci., 1885, p. 173.