374 



Aquatic Societies 



attached in the hollows of stones and crevices of rock 

 ledges. The case of the caddis-worm, Ithytrichia, (fig. 

 162 on p. 2(12) is broadly depressed. 



Thus the impress of environment is seen not only 

 in the form of a living animal but also in that of the 

 non-living shelter that it builds. In this there is a 

 parallel of form in the secreted shell on the back of the 

 snail, Ancylus, and manufactured shell on the back of 

 the caddis-worm, Helicopsyche. One would have to 

 search widely to find better examples of the effects of 

 environment in molding to a common form these 

 representatives of many groups of very diverse struc- 

 tural types. Two of them, at least, were sufficiently 

 like lotic mollusca to have deceived their original 

 describers. Psephenus was first described as a limpet 

 and Helicopsyche as a snail. 



Foraging habits — The food of the herbivores in lotic 

 societies is algae. There are none of the higher plants 



present, save a few 



r 



mosses of rather local 

 distribution. It is not 

 surprising therefore 

 that the food gather- 

 ing apparatus of these 

 forms should present 

 special adaptative 

 peculiarities. The 

 mouth-parts of may- 

 flies and of midges 

 show much develop- 

 ment of diatom rakes 

 and scrapers. For 

 scraping backward 

 the labrum is often 

 used. In the net-spinning caddis-worms it is bordered 

 on either side by a stiff brush of bristles, and in midge 



Fig. 225. The sheltering tubes of 

 midge larvae. Photographed under 

 running water on the rocky bed of a 

 stream. 



