140 INSECTS AND MAN 



to a rock or other support, just above the water, the while 

 the tips of her wings and abdomen vibrate in the water. 

 The eggs, to the number of two hundred, are laid in patches, 

 and adhere to some submerged support by means of a 

 gelatinous covering. After about a week, the time depends 

 on the temperature of the water, the larvae (fig. 38) emerge. 

 They are usually greenish brown in colour, and of a 

 peculiar club shape. Their chitinised heads are furnished 

 with two fanlike appendages, each composed of about fifty 

 filaments, attached to a stalk, which, by continual expansion 

 and contraction, draw food, consisting of minute aquatic 

 plants, into the gullet. Just below the head is a conspicuous 

 proleg furnished with a sucker, whilst at the posterior end 

 there is a large terminal sucker, by means of which the 

 larva remains attached in an upright position to some 

 support. But its habitat is below swiftly running water, 

 and a false step would mean destruction; so absolute 

 reliance is not placed on the sucker: a silken thread is 

 spun and affixed to some support, a life line, if you will, 

 by means of which the larva can remain at anchor. In 

 two or three weeks, or perhaps only after several months, 

 depending on the temperature and rapidity of the water, 

 the larva spins a characteristic silken cocoon in the form 

 of a pocket, open at its larger end (fig. 39). In this 

 curiously shaped dwelling it pupates, holding to its silken 

 home by means of abdominal spines. The head of the 

 pupa, which protrudes from the open end of the cocoon, is 

 furnished with a pair of tufted breathing filaments which 

 float about in the water. Towards the end of the second 

 or third week, a bubble of air gradually collects within the 

 cocoon, the pupa case splits, the fly emerges, and, rising 

 within the air bubble, reaches the surface of the water 

 without wetting its wings. Once in the air it darts away 

 immediately ; there is no wait for wings to dry or expand, 

 no hesitation, only an eager search for blood of man or 

 beast. 



