REACTIONS OF ORB-WEAVING SPIDER TO VIBRATIONS OF WEB. 321 



plitude with a vibration rate of 24 to 300 per second. It was 

 impossible with the materials at hand to construct a vibrator 

 giving a high rate and having also a considerable amplitude, so 

 recourse to steel wires and small forks was necessary. The large 

 spiders did not respond well to wires and forks with high vibra- 

 tion rate and small amplitude but they did respond instantly 

 to the vibrating wings of Chrysops (127 per sec.), Microbembex 

 (208 per sec.), Musca (284 per sec.), where the amplitude ranged 

 from 4 mm. to 10 mm. Small spiders responded quickly to 

 vibrations ranging from 100 per sec. to 487 per sec. and even 

 higher although the amplitude was very small. This difference 

 in responsiveness between the young and old spiders is probably 

 correlated with differences in size and rate of wing vibration of 

 the insects which are ensnared and used as food by young and 

 old. In general small insects have high wing vibration rates 

 while the larger insects have lower rates of wing vibration 

 (Packard, '03. p. 150). The smaller spiders eat small insects 

 and the large spiders eat larger insects. The following species 

 of insects were caught and eaten by E. sclopetaria: Chrysops 

 vitatus (127 vibr. per sec.); Calliphora vomitaria (130 vibr. per 

 sec.) ; Microbembex monodonta (208 vibr. per sec.) ; Musca domes- 

 tica (284 vibr. per sec.). Many small midges (Chironomus and 

 others) were eaten by the young spiders and occasionally by the 

 adults. The vibration rate of these small midges is probably 

 very high, judged by the high pitched note which they give out, 

 but it was impossible at the time to determine its rate. 



2. Experiments Using a Y-shaped Vibrator. 



In order to determine whether the spider reacted to a single 

 vibrating strand or to the center of a vibrating area of the web, 

 a Y-shaped vibrator made up of insulated magnet wire was 

 adjusted to the vibrator and arranged in such a manner that its 

 ends touched the web at two places, 2 or 3 cm. apart. When the 

 vibrator so adjusted was operated the spider responded readily, 

 going to a point on the edge of the web midway between the two 

 vibrating points and then after some slight hesitation going toward 

 one or the other of the vibrator wires (see Fig. 4). If, however, 

 these points of wire were more than 3 cm. apart the spider at the 



