II FLIES WITH AQUATIC LARVAE 195 



not buried in earth, but floating on water. The larva 

 can descend into the water when attacked, but the 

 pupa is too buoyant, and too much encumbered by 

 its outer case, to execute any such manoeuvre. Pro- 

 vision has accordingly to be made for the protection 

 of the helpless pupa against its many enemies. It is 

 probable that hungry Insects and Birds mistake the 

 shapeless larval skin, floating passively at the surface, 

 for a dead object. The considerable space between 

 the outer envelope, or larval skin, and the body of the 

 pupa may keep off others, for the first bite of a Dytis- 

 cus or Dragon-fly larva would be disappointing. Still 

 further security is gained by the texture of the larval 

 skin itself. The cuticle consists of two layers. The 

 inner is comparatively soft and laminated, while the 

 outer layer is impregnated with calcareous salts, and 

 extremely hard (Fig. 67, 7, 8). The needful flexibility is 

 obtained by the subdivision of the hard outer layer. 

 Seen from the surface, it is broken up into a multitude 

 of hexagonal fields, each of which forms the base of 

 a conical projection, reaching far into the softer layer 

 beneath. The conical shape of these calcareous nails 

 allows a certain amount of bending of the cuticle, 

 while the whole exposed surface is protected by an 

 armour, in which even the pointed mandibles of a 

 Dytiscus larva can find no effective chink. 



Swammerdam once found a pupa of Stratiomys 

 dead within its larval skin. In the abdomen were 

 larvae and pupae of a parasitic Insect (perhaps an 

 Ichneumon or a Tachina) which had sprung from eggs 

 laid within the Stratiomys. Neither water nor hard 

 skin can exclude these cruel and insidious enemies. 



O 2 



