THE FARM STREAM 37 



thread is exuded at the mouth (as a 

 liquid which hardens on contact with the 

 water), attached to the stone and spun 

 out to the desired length. The larva, with th^hiack^&y'^tsimuiiuml 

 disc loosened, swings free upon the thread, 

 reversed in position and hanging with head upstream. 

 After a time it will fasten itself by its sucker again. By 

 using a very short thread and its sucker alternately, the 

 larva may move short distances over the supporting surface 

 in a series of loopings, its position being reversed at each 

 attachment in a new place. Black-fly larvae are excellent 

 food for fishes, but they live for the most part in places that 

 are to fishes wholly inaccessible. They feed upon micro- 

 scopic organisms and refuse adrift in the stream, and they 

 gather their food out of the passing current by means of a pair 

 of fan-like strainers, located on the front of the head near the 

 mouth. Adiilt black-flies of certain species bite fiercely in 

 northern forests. Other species, known as "buffalo-gnats" 

 and "turkey-gnats", are important pests of live stock. Other 

 species are harmless. 



In the same situations with the 

 black-fly larvae, the neat little food- 

 traps of the seine-making caddis-worms 

 may always be found. Each is a little, 

 transparent, funnel-shaped net, half an 

 Fig. 18. Diagram of a inch wide. Opening always upstream, 

 Sn?apjfr?tus and'Til and tapcHng downward into a silken 

 thl^diVe^Aiorof ?he^?urrent tubc, lodgcd in somc sheltering crevice, 



over the stream bed ; a, the -^ . i,* 1 ii • i mi u 



front edge of the distended iH which the grccmsh, gill-beanng 



watir ^iJ^strainldrl the caddis-worm that makes it dwells. 

 mes&the^'bJtfom of the Then thcrc is a group of diverse in- 



"oor^ftttuir^in which scct larvac found habitually in the 



the larva dwells, in the „ • j i • • x j. j.i a. 



shelter of the rock, d. (After rapids clmgmg to stoucs, that agree 

 Mi^HceA.'NoyS"^^'^ in being flattened and more or less 



