338 



AMERICAN SPIDERS AND THEIR SPINNINGWORK. 



up the various lines of the snare, so that all the radii, particularly, are 

 held in a tense condition. 



This tension of the trapline and radial lines makes the whole web an 

 efficient telegraphic instrument for conveying to the spider in her den or 

 domicile any vibration caused by insects entangled upon the snare. When 

 by such telegraphy the capture of an insect is communicated to the sen- 

 sitive feet of the spider, she immediately rushes along her trapline to the 

 hub, and from that point to her prey. Thus, in the typical Orbweaver's 

 snare, the trapline serves the two purposes — first, of communicating to 

 the proprietor the presence of entangled insects; and, second, of affording 

 a gangway to the net and back again to the tent. In short, the incidental 

 dragline appropriated to the uses of a gangway, in Epeira strix, is here 

 specialized into a trail and a telegraph. 



As far as I can positively affirm, no other than these two purposes 

 are served by the trapline in these species. However, it is the habit of 



spiders who tlius use this specialized instru- 

 ment to frequently pull upon it, increasing the 

 tension by drawing it towards them- 



Jerking- selves and then letting it go again, 



the Trap- , . . p •-, • , t 



,. making a series of rapid jerks, i 



have never been able to observe that 

 this motion had any purpose or effect to in- 

 crease the entanglement of an insect, and have 

 always regarded it simply as a means of deter- 

 mining the presence, weight, and energy of the 

 victim, and thus estimating the degree of cau- 

 tion necessary in approaching it. If the insect 

 be quiescent, as often occurs, the pulling and 

 sudden relaxing of the trapline is quite sure to set it in motion again. 



It might, perhaps, be observed in this connection that even when the 

 spider is hanging on guard at the centre of her orb, she spins a series of 

 deltated lines, the apex of which is grasped by several claws of 

 the legs, and the opposite ends of which are attached to the 

 radii in such a way as to cause a direct communication between all parts 

 of the orb and the sensitive feet within which these lines are thus con- 

 verged. Examples of tliis are especially observable in the various species 

 of Acrosoma. Fig. 329 shows the manner in which Acrosoma rugosa, when 

 hanging upon the open hub of her snare, thus gathers converging radii 

 into her claws. A like habit in the cases of Acrosoma mitrata and spinea 

 is illustrated by Figs. 112 and 113, page 127. In the same manner that 

 the trapline is jerked by nest building Epeiras, in order to tighten it and 

 test the presence of insects, this series of footlines is also frequently jerked, 

 and apparently for the same purpose. 



Wliile I cannot record observations which justify, me in asserting that 



Fig. 329. Acrosoma rugosa hanging 

 at her hub, to show the radii clus- 

 tered on the claws. 



