i;o AMERICAN SPIDERS 



tralian region and ordinarily placed near the Oonopidae, are sed- 

 entary types similar to Enryopis that have become vagrant 

 secondarily. Although they have lost their unpaired claws, they 

 still retain spurious claws and numerous other features that point 

 to an origin from the comb-footed spiders. 



THE SHEET WEB WEAVERS 



The addition of a formal horizontal platform marked a signif- 

 icant departure from the irregularity of the tangled space web. 

 This strengthened zone of thin and loose webbing, with the egg sac 

 at its hub, quickly became the theme of a new type of snare in 

 which the upper and lower mazes and the guy lines now played a 

 subsidiary role. The germ of the platform was present in the webs 

 of some of the comb-footed spiders, but developed no further 

 there; the closely woven sheet of the sheet web weavers and the 

 geometric snare of the orb weavers, however, are its direct results. 

 These latter spiders represent a common stock that, though early 

 branching onto separate roads, has come down to modern times as 

 two closely allied lines. So much in common have these dominant 

 aerial spinners that they were for a long time classified within the 

 limits of a single large family. The comb-footed spiders diverged 

 from the line at nearly the same time, perhaps because of failure to 

 introduce regularity into their web by exploiting the platform, and 

 took a path toward perfection of lobed glands and tarsal combs. 



The sheet is a yielding table upon which drop flying and jump- 

 ing insects, usually after being halted in midair by a superstructure 

 of crisscrossed lines guyed to adjacent vegetation. The sheet web 

 weaver clings upside down beneath the blanket, runs over the sur- 

 face with rapidity, and pulls its prey through the webbing. The 

 principal sheet acts as an effective screen against enemies from 

 above, as well as a relatively efficient snare. A second sheet is often 

 present beneath the hanging spider, apparently serving as a barrier 

 to attack from below. The sheet webs (Text Fig. 5, A) are used for 

 a long time; when partially destroyed by winds or falling debris, 

 they are replaced after a few hours of spinning. In some instances, 

 the stopping maze above the trap is missing, or is represented only 

 by a few guy lines. Snares placed near the ground are effective in 

 stopping and holding collembolans and small insects of many types. 



The sheet web weavers of the family Linyphiidae far exceed in 



