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. 262) 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 

 structural types. Two of them, at least, were suffi- 

 ciently 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 alga?. There are none of the higher plants 



present, save a lew 



mosses of rather local 

 distribution. It is not 

 surprising therefore 

 that the food gather- 

 ing apparatus of these 

 forms should present 

 special adaptative pe- 

 culiarities. 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 sidejby 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. 



