194 NATURAL HISTORY OF AQUATIC INSECTS CH. 



vessel containing nothing but water their transforma- 

 tion was not hindered. The Insect becomes rigid ; 

 its skin hardens and contracts to some extent ; the 

 last segments become distorted. On opening a larva 

 which exhibits these marks of change, the animal will 

 be found to have shrunk and changed its shape. It 



now presents the form of the 

 future fly, but is shrouded in a 

 silky cocoon, the secretion of 

 the salivary glands, and occupies 

 only the fore part of the space 

 within the larval skin (Fig. 67, 4). 

 Some entomologists have even 

 taken the pupa to be a parasite 

 which had eaten up the larva of 

 the Stratiomys, and undergone 

 transformation in the empty 

 skin. 1 Remnants of the air-tubes 

 and the cast lining of the in- 

 testine moor the pupa, as it were, 

 to the tail of the larval skin. 

 The great reduction in size of 

 Stratiomys during transforma- 

 tion is an extreme case of what 

 may often be remarked in In- 

 sects. The larva gets some unknown advantage from 

 a length and bulk which would apparently be incon- 

 venient to the flying Insect. 



The peculiar structtire of the larval skin now 

 acquires a special value, especially when the pupa is 



1 Westwood, Classification of Insects, 1 1.. p. 532, quotes such 

 cases. 



FIG. 69. Pupa of Stratiomys, 

 removed from larval skin. 

 From Swammerdam,.Z>/$/;Vz 

 Natures. 



