I 62 METAMORPHOSIS INTEGUMENT CHAP. 



highest admiration. The greater part of the information ob- 

 tained refers to a single Insect, the blowfly ; and amongst those 

 who have made important contributions to it we may mention 

 Weismann, 1 Viallanes, 2 Ganin, 3 and Van Kees, 4 and it is at pre- 

 sent under investigation by Lowne. A good deal, too, is becoming 

 known about the processes in the case of the silkworm. 



Integument and Ecdysis. 



The integument consists of a cellular layer, usually called the 

 hypodermis, situated on a basement membrane. The" hypo- 

 dermis, or layer of chitinogenous cells, excretes a matter which 

 remains attached to the body, forming the hard outer layer of the 

 skin. This layer consists of chitin and has no vitality, but 

 its presence no doubt exerts a very important influence on the 

 physiological processes of the Insect. The chitinous investment 

 varies much in thickness and in other properties ; in some 

 Insects it is hard, even glassy, so as to be difficult to pierce witli 

 a pin, in others it is pliable, and in some very delicate. Chitin 

 is a substance very difficult to investigate ; according to the 

 recent researches of Krawkow 5 it may prove to be of somewhat 

 variable chemical composition. 



After a time the hypodermis excretes a fresh supply of 

 chitin, and, possibly by the commencement of this process, the 

 older chitinous investment becomes separated and is shed. The 

 details have, however, not been ascertained, though their import- 

 ance has been suggested by Hatchett Jackson. 6 The newly 

 exposed layer of integument is pallid, but afterwards becomes 

 coloured in a manner varying according to the species, the process 

 being possibly due to some secondary exudation permeating the 

 freshly exposed chitin, or modifying some part of its exterior. 



Lowne informs us that in the imago of the blowfly the great 

 majority of the hypodermic cells themselves enter into the com- 

 position of the chitinous integument ; and it is perhaps not a 

 matter for surprise that the cells should die on the completion of 

 their functional activity, and should form a part of the chitinous 



1 Zeitschr. wiss. Zool. xiv. 1864, p. 187. 



2 Viallanes, Ann. Sci. Nat., Series 6, "Zool." xiv. 1882. 



3 Unfortunately in the Russian language. 4 Zool. Jahrb. Abth. Anat. iii. 1888, p. 1. 



5 Zeitschr. Biol., xxix. 1892, p. 177. 

 6 Trans. Linn. Soc. London, "Zoology," 2nd series, v. 1890, p. 174. 



