BROOK, CREEK, AND RIVER 287 



This is attached to the rocks on the bottom. The large end of 

 the tube is covered with a cup-shaped net spun of silken strands. 

 The larva lies at one side of the net and picks off the small 

 animals and plants the current brings down. Not infrequently 

 the entire upper surface of the stones on the bottom will be 

 covered with these net tubes of this larva. Another caddis- 

 fly larva, Helicopsyche, has a coiled tube built of sand grains 

 (Fig. 444) that appears like a small snail shell. This lives in mod- 

 erately rapid water of the large creeks where the bottom is pebbly. 

 It attaches to the rocks. The larva of the black fly, Simulium, 

 is universally present in the rapid water running over stones 

 from the merest trickle in the tiny runnel way down stream as 



FIG. 442. Sharp-headed darter, Hadropterus phoxocephalus 



long as rocks on the bottom afford attachment. The larvae 

 appear like black, squirming, short worms with brushes of bristles 

 at the free end, by means of which they catch the tiny animals 

 and plants floating in the current. They are so numerous as to 

 make great patches on the rocks. Each larva is attached at 

 one end by a sucker but can let go at will when it spins a silken 

 strand, at the end of which it dangles in the water, seeking new 

 feeding ground. The adults are the pestiferous tiny flies that 

 swarm about the head of the fishermen or the udder of browsing 

 cattle and take bloody toll. Under the stones one finds blood- 

 worms, larvae of midges of genus Chironomus, stone-fly nymphs 

 (Perla}, damsel-fly nymphs (Argiaputridd), May-fly nymphs 

 (Siphlurus alternatus) , the larva of a water beetle (Fig. 446) so 

 flattened and rounded it is called the water penny (Psephenus 

 lecontei}. These nymphs in the rapidly running water have 

 head, body, and limbs all flattened to offer the least possible 



