METAMORPHOSIS 163 



investment. Some writers say that the chitinous layer may be 

 shown to be covered by a delicate extima or outer coat. 



The number of ecdyses varies greatly in Insects, but has been 

 definitely ascertained in only a few forms outside the Order 

 Lepidoptera. In Campodea Grassi says there is a single frag- 

 mentary moult, and in many Hymenoptera the skin that is cast 

 is extremely delicate, and the process perhaps only occurs twice 

 or three times previous to the pupal stage. In most Insects, 

 however, ecdysis is a much more important affair, and the whole 

 of the chitinous integument is cast off entire, even the linings of 

 the tracheae, and of the alimentary canal and its adjuncts being 

 parted with. Sir John Lubbock observed twenty-three moults 

 in a May-fly of the genus Cloeon, 1 this being the maximum yet 

 recorded, though Sommer states 2 that in Macrotoma plumbea- 

 moulting goes on as long as life lasts, even after the Insect has 

 attained its full size. 



Some Insects get quit of a considerable quantity of matter by 

 their ecdyses, while in others the amount is comparatively slight. 

 It has been thought that the moulting is effected in order to 

 permit of increase of size of the Insect, but there are facts which 

 point to the conclusion that this is only a factor of secondary 

 importance in the matter. One of these is that many Insects 

 make their first ecdysis almost immediately after they leave the 

 egg ; this is the case with the young larva of the blowfly, which, 

 according to Lowne, moults within two hours of its emergence 

 from the egg. We have already referred to the important sug- 

 gestion made by Eisig 3 that, since chitin is a nitrogenous sub- 

 stance, the ecdyses may be a means of getting rid of waste 

 nitrogenous matter ; to which we have added that as chitin also 

 consists largely of carbon, its excretion may be of importance 

 in separating carbonaceous products from the blood. 



Metamorphosis of Blowfly. 



The phenomena of metamorphosis are displayed to their 

 greatest extent in the transformations and physiological processes 

 of the Muscid Diptera, of which the common blowfly is an 



1 Trans. Linn. Soc. xxv. 1866, p. 491. - Zeitschr. wiss. Zool. xli. 1885, p. 712. 



3 "Fauna und Flora d. Golfes von Neapel, " Z>w Capitdliden, 1887, p. 781. 



