136 NATURAL HISTORY OF AQUATIC INSECTS CH. 



the pupa. This appears to remain plastic for a 

 long time, for it ultimately takes very exactly the 

 shape of the imaginal parts which form within it. 

 There is afterwards developed within it a second and 

 inner cuticle, which will ultimately form the integu- 

 ment of the fly. The two structures are always 

 distinct, but they are in most places in close con- 

 tact, and can only be distinguished by the micro- 

 scope. The pupal and imaginal cuticles do not at 

 all closely follow the larval skin, but become at 

 particular places folded far into the interior. The 

 folds which give rise to the head of the fly are two 

 in number and paired. They begin at the larval 

 antenna on each si3e of the head, and gradually ex- 

 tend further and further backwards. The object of 

 the folds is to provide an extended surface which 

 can be moulded, without pressure from surrounding 

 objects, into the form of the future head. On one 

 part of each fold the facets of the large compound 

 eyes are developed, another part gives rise to the 

 future antenna, a large and elaborate organ, which 

 springs from the bottom of the fold, and whose tip 

 just enters the very short antenna of the larva. The 

 folds for the head ultimately become so large that the 

 larval head cannot contain them, and they extend far 

 into the prothorax. Here a difficulty occurs. If the 

 generating cuticle of the prothorax were also to be 

 folded inwards, the future prothorax would take a corre- 

 sponding shape. But the prothorax of the fly has a 

 form dictated to it by the limbs which it bears, and 

 by the muscles to which it gives attachment. These 

 call for a great reduction in its length, and a peculiar 



