cook: web-spinning fly larvae 



191 



The only familiar objects to which the webs can be compared are 

 the rope signals that are hung near bridges and railroad tunnels 

 to avoid accidents to train crews. The 

 construction is simple but rather exten- 

 sive, the webs being usually over a foot 

 long and sometimes nearly 2 feet. Usu- 

 ally the same general direction is kept, 

 along the roof of the cave, but some- 

 times there is a simple curve and return. 



The whole structure is supported from 

 the roof of the cave by a few perpendicu- 

 lar strands, rather irregularly spaced, 

 usually about 2 inches long, and often 2 

 or 3 inches apart. The ends of these 

 supports are connected by a horizontal 

 cable. Where the roof of the cave is un- 

 even the lengths of the supports are 

 varied, so as to maintain the horizontal 

 direction of the cable. The ends of the 

 cable are drawn up and attached to the 

 roof, and there is only a little sagging 

 between the supports. The remainder 

 of the web consists of a fringe of perpen- 

 dicular threads attached to the cable 

 above and with the lower ends hanging 

 free. The threads of this fringe are 2 or 

 3 inches long, and from about 1 mm. to 

 3 mm. apart. A diagram, kindly pre- 

 pared by Mr. W. E. Chambers of the 

 Bureau of Plant Industry, is shown in 

 figure 1, to illustrate the plan and ap- 

 pearance of the web. The drawing 

 shows a small section of the fringe with 

 a part of the horizontal cable and one 

 of the vertical supports. 



The cable and its supports were very 

 slender and had the appearance of or- 



Fig. 1. Diagram of web 

 of cave dwelling fly larva. 



