io 4 AMERICAN SPIDERS 



and actually caused the divergence of the true spiders from the 

 parent line. 



It was inevitable that, in addition to the formal silken covering 

 over the egg mass, many threads would be scattered more or less 

 haphazardly from this spinning center. Such wild lines were in- 

 strumental in giving to the mother spider another advantage in her 

 efforts to guard the eggs, communicating the approach of an inter- 

 loper by vibrations on the threads. The range of touch perception 

 was thus in one step expanded far beyond mere contact with the 

 sensory hairs on legs or body; the deadly predator or the blunder- 

 ing insect often became the prey of the vigilant spider. In this two- 

 dimensional maze of threads, with the egg sac as central theme, was 

 the germ of all the webs that have made the true spider dominant. 



The stringing-out of silken lines continued during the whole life 

 of the spider, as well as at the egg laying, and has continued to the 

 present time as the dragline habit of modern spiders. With a secure 

 line attached to the spinnerets, the spider could now venture upon 

 precipitous surfaces with a certainty of quick recovery from falls. 

 Since the dragline of true spiders is ordinarily spun through the 

 anterior lateral pair, the tarantulas, in suppressing these spinnerets, 

 virtually precluded the future possibility of becoming aerial spiders. 



The lifeline of the whole group of true spiders became their 

 silken threads, and those that refused to accept subservience to this 

 material died out. Every spider became sedentary to a degree, and 

 none has been able to divest itself completely from silk since those 

 early days. Each major group of spiders diverged from the others 

 with essentially the same type of spinning equipment, and with a 

 well-founded instinctive knowledge of silk spinning. In each of the 

 lines similar types of webs and traps for the capture of insects have 

 been evolved separately. 



In one group the anterior median spinnerets have been perpet- 

 uated in a modified form as the cribellum. These creatures come 

 down to modern times in a more or less homogeneous line as the 

 "cribellate" spiders (p. 137). The whole series probably diverged 

 quite early from the main stem, and, although their physical fea- 

 tures mark them as a more generalized group, they have done re- 

 markable things with their heritage. All the remaining true spiders 

 lost the anterior median spinnerets, but in most of them vestigial 

 evidences can still be observed. 



During the early history of the ecribellate true spiders, a trend- 

 already running a similar course among the cribellate types toward 



