AQUATIC INSECTS 913 



ceridae) live in rapid streams, and are peculiar in the very strongly 

 depressed form of both larva and pupa, and in the row of ventral 

 sucking discs which the larva has developed for hanging fast. 



The black-flies (Simuliidae) also live in running water. The 

 larvae adhere to stones and timbers by a single sucking disc at 

 the hinder end of the flask-shaped body, which thus hangs sway- 

 ing in the current with head downstream. 

 Above the mouth on the front of the head 

 there are two processes which bear the name 

 of "fans." These are composed of a very 

 large number of scythe-shaped rays fringed 

 along the side. Set at an angle upon the 

 pedicel, hke the fingers of a reaper's cradle 

 upon the handle, together these constitute - — .^ 



a net for retaining small organisms adrift ''fory'disc It'Vsrie-flyTa™; 

 in the water, and for holding them up to setedon. 

 the mouth. This is an aboriginal plankton apparatus. Simulium 

 larvae play in the rapids, spinning silken threads in the water, and 

 swinging on them from place to place. Occasionally the threads 

 thus spun in the troughs of fish hatcheries have been sufficiently 

 numerous to entangle and kill newly hatched trout. These threads 

 are spun from the saUvary glands; a final use for the secretion of 

 these glands is the making of the open-meshed half-cornucopia- 

 shaped cocoon in which, attached to the sides of the rocks or tim- 

 bers, the pupal stage is passed; a branched prolongation of the 

 tracheal lining of the prothoracic spiracles constitutes the so-called 

 "tube gills," by means of which the black-fly pupa is able to 

 get its air supply while wholly submerged. 



The soldier flies (Stratiomyiidae) live as larvae on the surface of 

 still water. They float stiff and rigid and stick-like, with a circlet 

 of water-repellent bristles surrounding the terminal spiracles, keep- 

 ing open the way to the air. The pupa is formed within the 

 larval skin, without further outward indication of the change than 

 a slight angulation of the latter posteriorly. The adult soldier 

 flies hover familiarly about the arrow heads on shore at egg- 

 laying time, and at other times frequent flowers to feed on their 

 nectar. 



