AQUATIC INSECTS 



901 



ous spiral band; the larva of Helico psyche builds out of sand grains 

 a spirally coiled case, shaped like a snail shell. The materials 

 of the case are always stuck together by means of the secretion of 

 the salivary glands. Usually the cases are cylindrical but sometimes 

 they are triangular, or square in cross-section. Usually the sticks 

 used are placed lengthwise, but sometimes crosswise, as in stick 

 chimneys, to make the bulky and cumbersome dwellings of some 

 of the Limnophilidae. Sometimes, on the other hand, they are con- 

 structed so light and thin as to offer little hindrance to free loco- 

 motion, and a few larvae with well-developed swimming fringes on 

 their long oarlike feet swim freely 

 about. In the cases that are con- 

 structed by most larvae of the two 

 families Hydroptilidae (Fig. 1372) 

 and Rhyacophilidae, no extraneous 

 materials are used, but only the se- 

 cretion of the salivary glands; these 

 cases are therefore thin and parch- 

 ment-like. Most members of the 

 great family Hydropsychidae make 

 no portable cases at all, but only 

 runways in the crevices between the 

 stones in streams; these they line 

 with silken threads. Some of these 

 larvae, among which are the com- 

 monest members of the genus Hydro- 

 psyche, to be found in every swift 



, 1 r 1 Ti Fig. 1372. Micro-trichoptera. On the 



stream, Spm webs OI open mesn, like left, a lami; on the right, a pupa oi another 



. r ^ smaller species, within its transparent case. 



fishermen s semes, out from the up- 

 stream ends of their tubes or runways; clearly, this is for the 

 purpose of catching any little organisms set adrift in the stream. 

 These are mainly carnivorous larvae; many members of other 

 famines have a mixed diet of vegetable and animal food, but a 

 goodly number are characteristically herbivorous. 



There are caddisfly larvae for all sorts of waters, and lor wet 

 situations, or mossy banks. A few species, accompanying the 

 "blood worms," have migrated far out on the bottoms of our larger 



