V CADDIS-WORMS 263 



plish this it turns round in its case, and works with its 

 head and legs projecting from the narrow end till 

 a hole is made of suitable size to receive the case, 

 after which it resumes its ordinary position. The 

 cases are commonly secured by silk threads to stones 

 or other objects. 



De Geer describes the process of pupation and 

 the escape of the fly. 1 Having found cases just 

 closed with silken sieves, he opened one, and found 

 within it a larva not yet transformed. It had, how- 

 ever, lost the power of moving its limbs, which 

 seemed paralysed, though the abdomen was capable 

 of vigorous movement. 2 Next day the larval skin 

 was cast. The pupa now exhibited all the parts of 

 the imago, enveloped in a pupal skin, but not glued 

 down, as in a Lepidopterous chrysalis. The antennae 

 and legs were free, a point of importance in con- 

 nection with the emergence of the imago. 



The fly does not emerge beneath the water, adds 

 De Geer. The pupa first extricates itself from its case 

 by the help of the hooks described above. Then it 

 climbs up the stones or aquatic plants until it gains 

 the air, attaches itself by the claws of its legs, and 

 throws off the pupal skin. The pupa, when liberated 

 from its case, swims and runs about with surprising 

 agility. Swimming is mainly effected by the pair of 



1 Mem. sur les Insectes, u, 546, 516. 



The mobility of the abdomen of the pupa is a point of 

 practical importance. The respiratory movements kept up by 

 the abdomen of the larva are equally necessary in the next 

 stage, and the pupa has the lateral fringe of hairs, the tracheal 

 gills, and every other requisite for maintaining a flow of water 

 through its case. 



