192 cook: web-spinning fly larvae 



diiiary spider-webs, but the threads that formed the pendant 

 fringe were much thicker, perhaps 0.5 mm. in diameter, and ap- 

 peared as though filled or heavily coated with water. The thicken- 

 ing of the threads did not reach the junction with the cable, but 

 began about 5 mm. below, with great regularity. 



The constiuction of such a web implies, of course, the possession 

 of a highly specialized spinning instinct. Indeed, without observ- 

 ing the operation it is not easy to understand how the webs are 

 built unless we suppose that at least the supporting framework 

 of the structure is first laid out on the ceiling of the cave, to be 

 dropped into the pendent position afterward, perhaps when the 

 heavy fringe is added. But even on this assumption the provi- 

 sion for keeping the cable horizontal by varying the lengths of the 

 supports would involve a high order of instinctive skill. The 

 stretching of the cable by carrying a thread along the wall would 

 not seem so difficult, but more talent would be required to carry 

 the supporting threads up to the ceiling from the cable or to let 

 them down from above to meet the cable. 



\^Tien the pendent threads were gathered upon the finger they 

 formed a mass of slime, which shows that the material is very 

 unlike the silk of spiders. Yet the webs evidently serve the same 

 purpose of trapping insects. Several small insects were found em- 

 bedded in the shme, from which they could be squeezed out by 

 slight pressure. Mosquitoes and other soft-bodied forms, which 

 have the habit of seeking dark roosting-places, probably furnish 

 most of the victims, but one of the webs had caught a small beetle. 

 If an investigation of the insect life of the caves were to be under- 

 taken, these webs might afford considerable assistance in trapping 

 the small insects that flit along the roofs of the caverns. 



The larvae which were evidently the builders of these curious 

 structures, were slender, transparent, vermiform creatures about 

 20 mm. long. They were found in all cases lying along the main 

 cable of the nest, on which they seemed to slide back and forth, 

 with considerable speed. 



The attention of Mr. H. S. Barber, of the Bureau of Entomology 

 who visited Guatemala in the same season, was called to these 

 webs and he saw some of them in another cave near Trece Aguas. 



