258 Adjustment to Conditions of Aquatic Life 



as the larvae of midges and caddis-flies would indicate 

 that the habit has been biologically profitable. 



According to Betten the habit probably began with 

 the gathering and fastening together of fragments for a 

 fixed shelter, and the portable, artifically constructed, 

 silk lined tubes of the higher caddis-worms are a more 

 recent evolution. 



IV 



Withstanding the wash of moving waters Where 

 waters rush swiftly, mud and sand and all loose shelters 



FIG. 158. Stone from a brook bed, bearing tubes of midge 

 larvae and portable cases of two species of caddis- worms. 

 The more numerous spindle-shaped cases are those of 

 the micro-caddisworms of the genus Hydroptila. For 

 more distinct midge tubes see figs. 134 and 223. 



are swept away. Only hard bare surfaces remain, and 

 the creature that finds there a place of residence must 

 build its own shelter, or must possess more than ordi- 

 nary advantages for maintaining its place. The gifts of 

 the gods to those that live in such places are chiefly 

 these three: 



I. Ability to construct flood-proof shelters. Such 

 are the fixed cases of the caddis-worms and midge larvae 

 (fig. 158) to which we shall give further consideration in 

 the next chapter. 



