Plancton Gatherers 367 



The larva lies in its tube in readiness to seize anything 

 the current may throw down upon its feeding surface 

 or entangle in the sides of its net. The whole net is so 

 delicate that it collapses on removal from the water. 

 To see it in action, it is best examined through a 

 "water-glass."* 



Brachycentrus, the "Cubist" caddis-worm, is re- 

 stricted in habitat to spring-fed streams flowing 

 through upland bogs. It constructs a beautiful case 

 that is square in cross- section. Each side is covered 

 with a single row of sticks (bits of leaf stalks, grass 

 stems, etc.) placed crosswise. The larva fastens its 

 case by a stout silken attachment to the top of some 

 current-swept boulder and then rests with legs out- 

 spread as indicated in figure 217 in a receptive atti- 

 tude, waiting for whatever organic materials the current 

 may bring within its grasp. 



The Nymph of Chirotenetes , the ' ' Howdy' ' Mayfly, lives 

 on the rock ledge or where the water sweeps among the 

 stones. Its body is of the stream-line form discussed 

 in the last chapter the form best adapted to diminish- 

 ing resistance to the passage of water, as well when at 

 rest as when swimming. The nymph sits firmly on its 

 middle and hind feet. Holding its front feet forward, it 

 allows the current to spread out their strainer-like 

 fringes of long hairs. These retain whatever food is 

 swept against them, and the mouth of the nymph is 

 conveniently near at hand. It uses its feet for stand- 

 ing but moves from place to place by means of swift 

 strokes of its finely developed tail fin, supplemented by 

 synchronous backward strokes of its strong tracheal gill 

 covers. It has almost the agility and swiftness of a 

 minnow. 



*A "water-glass" is any vessel having opaque sides and a glass bottom, of 

 convenient size for use. An ordinary galvanized water pail with its bottom 

 replaced by a circular glass plate set nearly flush, is excellent. 



