FIELD BOOK OF INSECTS. 



a number of threads, become visible on the white ground. 

 These threads are in general stuck all over with small 

 vegetable particles, like fine dust, which make them much 

 more apparent. The threads extend in all directions 

 from leaf to leaf, and the larva has access to a perfect 

 labyrinth, along which it can travel to a fresh place by help 

 of the current and with the speed of lightning. . . . Al- 

 though the larva commonly slides along a thread previously 

 made, and easily seen to be an old one by the small parti- 

 cles which cling to it, it can upon a sudden emergency 

 spin a new thread, like a spider or a Geometer larva. . . . 

 When the time for pupation comes, special provision 

 has to be made for the peculiar circumstances in which the 

 whole of the aquatic life of the Simulium is passed. An 

 inactive and exposed pupa, like that of Chironomus, 

 may fare well enough on the soft muddy bottom of a slow 

 stream, but such a pupa would be swept away in a moment 

 by the currents in which Simulium is most at home. 

 Before pupation the Insect constructs for itself a kind of 

 nest, not unlike in shape to the nests of some Swallows. 

 This nest is glued fast to the surface of a water-weed. 

 The salivary glands, which furnished the mooring-threads, 

 supply the material of which the nest is composed. Shel- 

 tered within this smooth and tapering cocoon, whose 

 pointed tip is directed up-stream, while the open mouth 

 is turned down-stream, the pupa rests securely during 

 the time of its transformation. When the cocoon is first 

 formed, it is completely closed, but, when the Insect has 

 cast the larval skin, one end of the cocoon is knocked off, 

 and the pupa now thrusts the fore-part of its body into 

 the current of water. The respiratory filaments, which 

 project immediately behind the future head, just as in 

 Chironomus, draw a sufficient supply of air from the 

 well-aerated water around. The rings of the abdomen 

 are furnished with a number of projecting hooks, and as 

 the interior of the cocoon is felted by silken threads, the 

 pupa gets a firm grip of its cocoon. If it is forcibly dis- 

 lodged a number of the silken threads are drawn out from 

 the felted lining. 



"A serious difficulty now appears. The fly is a delicate 

 and minute Insect, with gauzy wings. How does it escape 



244 



