THE AERIAL WEB SPINNERS 173 



ning over it quite actively when shaken from their tiny webs. They 

 come to light chiefly when leaves, moss and organic debris are 

 sifted over a sheet by the careful collector. The erigonids are well 

 known for their aeronautic habits; in autumn they constitute a 

 large part of the total group of fliers. Since most of them are less 

 than one tenth of an inch long, they can fly in adulthood as well 

 as in the younger stages. 



The small descriptive information given these spiders reflects an 

 incomplete knowledge of their habits rather than their importance, 

 since they are represented by a large number of genera and species. 

 A high percentage of the spiders of the northern hemisphere, as 

 well as most of the hardy boreal types that penetrate far into the 

 cold north and frequent the tops of our highest mountains, belongs 

 to this group. Their tiny flat webs, fortified with a dense covering 

 of viscid droplets, must reap a tremendous harvest of tiny insects 

 to maintain such a population. 



Some of the best-known members of this series belong in the 

 genus Erigone, which includes numerous dark brown or black spi- 

 ders with smooth and shining carapaces armed on the sides with 

 heavy teeth. The chelicerae and the pedipalpi are likewise often 

 studded with sharp spines. These erigonids are frequently found 

 along the edges of streams or lakes, where they place two-inch- 

 square webs among the grass roots or suspend them across stems 

 over the water. 



Many male erigonids have heads pitted and modified into gro- 

 tesque shapes. A slender horn, somewhat thickened at the end and 

 set with rows of stiff hairs, extends forward between the eyes of 

 Cornicularia. In Gnathonargus unicorn a single, long, slender horn 

 projects from the middle of the clypeus. A rounded lobe carries the 

 posterior median eyes of Hypselistes florens, and of many similar 

 species, high above the remaining pairs. One of the most amazing 

 of all erigonids is the European Walckenaera acuminata, whose eyes 

 sit in two groups at the top and middle of a slender tower more 

 than twice the height of the head itself. Often associated with these 

 bizarre modifications are deep, conical pits usually placed just back 

 of the posterior lateral eyes. The use to which such pits are put ap- 

 pears to be known only for the European Hypomma bituberculata. 

 W. S. Bristowe noted that during mating the female seized the male 

 by the head and inserted the claws of her chelicerae into the 

 rounded pits. This observation suggests that many other species 

 with pitted heads may perform in a similar manner, and further, 



