n FLIES WITH AQUATIC LARV/E i8i 



with small vegetable particles, like fine dust, which 

 make them much more apparent. The threads ex- 

 tend 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. I suppose that it grasps 

 the thread with its prothoracic claws, for when it 

 comes to rest it is always found holding on by them. 

 To recover its first position is not difficult if the net- 

 work of threads is intact, or if the larva has even a 

 single thread to grasp. Sometimes it hauls itself up 

 hand over hand, like a Leech or a Looping caterpillar, 

 applying its two suckers alternately to the thread. 

 It can also creep along its thread by means of the 

 prothoracic hooks only. I have often seen the body 

 swept into a position at right angles or nearly so 

 with the thread ; then the larva seems to be holding 

 fast by its head. When thus attached, it can travel 

 against the stream at a slow but quite appreciable 

 rate. 



Although the larva commonly slides along a thread 

 previously made, and easily seen to be an old one by 

 the small particles which cling to it, it can upon a 

 sudden emergency spin a new thread, like a Spider 

 or a Geometer larva. The new threads are perfectly 

 clean, and consequently invisible even on a white 

 ground so long as they are submerged, I got proof 

 of the formation of fresh-spun threads by dislodging 

 larvae which had attached themselves to leaves wiped 

 clean a minute before. But the clearest proof is got 

 by suddenly lifting out of the water a leaf with many 

 larvae upon it. One or two are pretty sure to let 



