264 NATURAL HISTORY OF AQUATIC INSECTS CH 



intermediate legs, which arc broad and fringed with 

 hairs. Sometimes the fore legs are fringed also- 

 Fritz Miiller remarks that certain pupa; of Caddis- 

 flies, which are not submerged, want these hairy 

 fringes, an interesting verification of the interpretation 

 commonly accepted.^ The pupa commonly swims 

 back downwards. Some of the smaller species do not 

 creep out of the water, but float to the surface, and 

 the fly in such cases escapes from the floating pupa 

 skin after the manner of a Gnat. 



The dorsal surface of the pupal abdomen commonly 

 bears a number of hooks or groups of hooks, which 

 are no doubt used, like the very sim.ilar hooks of 

 many Lepidopterous and other pups, for movement 

 within the cocoon. 



Some mention should be made of the Caddis- 

 worms which do not carry about cases. These belong- 

 to the families Hydropsychida; and Rhyacophilidai, 

 and are found usuall}^ in rapid streams. They have 

 retreats, often common to several larvae, which are 

 fixed and not portable, often consisting of stones 

 fastened by silk threads to one another and to a 

 larger stone. The larva; seek their food abroad, 

 lurking in crevices. They are sometimes, if not 

 always, carnivorous, preying upon Ephemerae, Simu- 

 Hum larvae, and the like. In these larvee the lateral 

 fringe of hairs upon the abdomen and the retractile 

 processes of the first abdominal segments are wanting, 

 a confirmation of the explanation given of these struc- 

 tures, viz., that they aid in promoting a stream of 

 water through the confined tube of the ordinary 

 1 Nature^ March 20, 1879. 



