REACTIONS OF ORB-WEAVING SPIDER TO VIBRATIONS OF WEB. 323 



least affected. If there is any motion here it is probably at right 

 angles to the original vibration, that is, it is probably parallel 

 to the spiders' long axis after orientation. 



5. Mutilation Experiments. 



The foregoing experiments coupled with careful observations 

 on the spinning behavior of the orb-weaver lead to the conviction 

 that the organs used in detecting the movements of the web 

 are proably tactile, at least there are no other organs described 

 which would seem to serve the purpose as well. There can be 

 little doubt that sense hairs are very abundant on the legs, par- 

 ticularly on the tarsi of these spiders. These hairs have been 

 described by Dahl (83), Wagner (88), McCook (90), and recently 

 by Mclndoo (n). The functions of these hairs have been inter- 

 preted in various ways, but little or no experimental work has 

 been accomplished other than attempts to show that some spiders 

 hear. Responses to sounds seem to have been observed only in 

 those forms which build webs. It seems likely that responses 

 in the web building forms are due to the vibrations of the air 

 being picked up by the strands of the web (Mclndoo, 'n, p. 412). 

 It was thought desirable to determine if possible the location of 

 the sense-organs used in detecting vibrations. By careful manip- 

 ulation with a pair of fine dissecting scissors it was possible to 

 snip off one or more of a spider's legs without causing the spider 

 to leave the web. It is necessary to use great care not to shake 

 the web because an irregular shaking gives rise to the negative 

 response, the spider running away to the retreat. The contrast 

 between this insensibility to the amputation of legs and extreme 

 sensitiveness to irregular vibrations of the web emphasizes the 

 fact that these spiders receive most if not all of their mechanical 

 stimuli through the web. These operations caused the spider to 

 lose considerable blood but two or three hours usually sufficed 

 to heal the wound. The stumps of the legs were always held up 

 so that they did not touch the web. 



Experiment i. After testing a spider to be assured that its 

 responses were normal the two forelegs were cut off as near the 

 middle of the metatarsus as possible. This spider immediately 

 put the stumps of the forelegs into its mouth. The next morning 



