146 • TRICHOPTERA. 



which has been bred by Mr. Parfitt, the case appears to be a 

 simple silken tube, without any covering of sand, and has 

 considerable resemblance to the shells of some molluscous 

 animals ; and I may remark here, that Trichopterous cases 

 have more than once been described as shells, and this I 

 believe in one instance by a conchologist still living. 



It may be as well to mention, that, although I have spoken 

 of cases in this group as fixed, they are probably never 

 really so until the larva is about to change to a pupa ; though, 

 as many of these insects inhabit running waters, they tempo- 

 rarily fix their cases, but at the same time they are perfectly 

 able to shift their positions. 



The insects of the second group, comprising the families 

 Mhyaco'philidce and Hydro'psuchidce, display much less 

 ingenuity in the method of constructing their cases; in fact, 

 it is probable that many of them make no case at all until the 

 time approaches when the larva is to change, and that during 

 the greater part of their larval existence they live free under 

 stones, sometimes making slightly constructed habitations, 

 into which they retire on the approach of danger. When 

 full grown, the larvae make clumsy looking cases of the 

 following description. Finding a suitable position, frequently 

 in the cavities in a large stone, they cover themselves with a 

 silken dome, to the exterior of which they attach small 

 angular stony fragments, so that externally they have the 

 appearance of irregular, more or less oblong, heaps of small 

 stones, and from this point of view it is almost impossible to 

 determine whether a case of this sort be that of an insect of 

 the family Rkyacophilklce or Hydroimychidcc. There is, 

 however, an infallible rule by which to separate them, viz., 

 that whereas the larvae of the first family construct an 

 envelope of brown silk in which they change to pupae, the 

 larvae of the Hy dropsy ch'idce change, without any special 



