86 THE LIFE-STORY OF INSECTS [CH. 



retain the power of mutual motion than in those 

 pupae which do not come out of their cocoons. 



While the pupa on the whole resembles the imago 

 that is to emerge from it, there are not a few cases 

 in which a special structure necessary for some 

 contingency in pupal life is retained or adopted in 

 this stage. A butterfly pupa, like the imago, has 

 no mandibles, but in the case of the Caddis-flies 

 (Trichoptera) and two families of small moths, the 

 most primitive of all Lepidoptera, the pupa, like the 

 larva, has well-developed mandibles. These enable 

 the caddis pupa to bite its way out of the shortened 

 larval case in which it has pupated, and then to swim 

 upwards through the water ready for the caddis-fly's 

 emergence into the air. Pupae that are submerged 

 require special breathing-organs. In the previous 

 chapter (p. 77) mention was made of the gnat's aquatic 

 larva with its tail-spiracles adapted for procuring 

 atmospheric air through the surface-film. The pupa 

 of the gnat 1 also has 'respiratory trumpets' serving 

 the same purpose, but these are a pair of processes 

 on the prothorax, so that the pupa, which is fairly 

 active, hangs from the surface-film with its abdomen 

 pointing downwards through the water. This change 

 of position is correlated with the necessity for the 

 imago to emerge into the air ; were the pupa to hang 

 head downwards as the larva does, the gnat would 



1 See Frontispiece, B. 



