i82 NATURAL HISTORY OF AQUATIC INSECTS CH. 



themselves drop a foot or more in the air, and the 

 thread can be seen to glisten in the sun, and to 

 lengthen itself at the pleasure of the Insect. The 

 Simulium larva, possessing this power of instantan- 



FIG. 63. Late larva of Simulium, showing changes within the larval skin, fl, fi, fi, 

 fore, middle and hind legs of fly ; ra, respiratory appendages of pupa ; ?u, wing 

 of fly; /i, haltere of fly. 



eously manufacturing a rope, can hardly be taken at 

 a serious disadvantage. 



The salivary glands, which secrete the silk, are 

 unusually large in this larva. They extend the whole 

 length of the body, and then bend forwards for a 

 third of its length. The tracheal tubes are large, 

 and give off a network of fine branches to the skin. 

 These appear to absorb oxygen from the water, for 

 there is no opening into the tracheal system. 



When the time for pupation comes, special pro- 

 vision has to be made for the peculiar circumstances 



