FAMILY RHYACOTHILIDy^ 



The insects of this group have a broader distribution and 

 more of them extend to the south than any of the preceding 

 families. The flies are small or of medium size. The larvae are 

 found in rapid streams and, instead of making free cases with 

 which to move about, their cases are formed of small stones 

 fastened with silk to the rocks — a wise provision, considering the 

 rapidly-running character of the streams which they inhabit. 

 These cases are very slight and loosely formed — merely a few 

 pebbles fastened to the lower surface of a large stone by a few 

 threads of silk, but, living on the under surface of rocks as it does, 

 the insect is less liable to be attacked by fish or predatory insects, 

 and does not need so elaborate a case as do the free swimmers or 

 crawlers. When it transforms to pupa a special protection is 

 formed within the case in the shape of a brown cocoon, which 

 is thin but of tough texture. We have a few widely-distributed 

 species in this family, which belong to four genera. 



