THE SENSES AND THEIR ORGANS. 



311 



but thinks that the functions of these three types are not identical, since 

 being found upon the same individual, one cannot well admit that three 

 different organs are constructed for the same physiological role. Might 

 they not, however, serve for different degrees of the same function? 



Wagner does not doubt that spiders have a delicate sense of hearing, 

 but the objective ground on which he rests it is unreliable. He states 

 it as "a fact known to all biologists," that in order to entice a spider 

 from its nest or den it is only necessary to cause a fly to buzz near it, 

 while an unskillful imitation of the 

 buzzing sound fails to deceive the ara- 

 nead. On the contrary, even the pres- 

 ence of flies in the web often fails to 

 tempt tlie spider forth ; and I know 

 that unskillful imitations of insects have 

 often drawn tliem forth ; but such imi- 

 tations I have never confined to sounds. 

 They are only or chiefly successful when 

 the movement is communicated to the 

 threads of the web itself. 



Wagner admits that if the movement 

 of hairs of any type under the influence 

 of sound could be proved, that would 

 suffice to assign to that type the role of 

 an auxiliary auditory organ, at least. 

 But, in fact, it is far from being proved 

 that sound sets the so called auditory 

 hairs into movement. At least his own 

 experiments failed to show this. By the 

 aid of an electric lantern he was able to 

 throw distinctly upon a screen figures of 

 a row of auditory hairs, enlarged to the 

 size of from three to six inches. He had 

 prepared a fine section of that portion of 

 the foot which is provided with auditory 

 hairs, and this had been so placed as to allow free movement. Sounds of 

 various sorts and tones were then produced without having any effect upon 

 the shadows on the screen. 



Suspecting that his lack of success might be due to the dryness of his 

 preparation, he replaced it with a foot freshly cut from a living spider. 

 The figures of the hairs were thrown clearly upon the screen, and, again, 

 every effort to cause them to move by the aid of sound waves was unsuc- 

 cessful. The size of the hairs upon the screen was so considerable that 

 the least vibration would be perceived, and, tlierefore, he could hardly ad- 

 mit that the failure was the result of defect in his experiment. From the 



Fig. 295. Section of a Beaded hair. The parts 

 corresponding to those of the Tactile hair are 

 marked by the same letters, z, an eminence 

 around the superior part of the basal thicken- 

 ing of the fold by which the free borders are 

 bent against the stalk of the hair, and form a 

 little external pouch which is situated in front 

 of the large e.xternal pouch ; bo.e., cavity of the 

 large external pouch; r, root of the hair vtith 

 its thickening ; p.t.r., section of the walls of the 

 external pouch by which the inferior part of 

 the basal thickening of the fold (t.r.s.) is set 

 below ; b, wall of the large external pouch, cor- 

 responding to the annular elevation of the first 

 layer of chitine in a Tactile hair, b, Fig. 294. 



