THE GEOMETRICAL SNARE 115 



how acute is the delicate sense of touch. Watch a 

 spider seated at the centre of its snare. Its sensitive 

 limbs diverge so as to rest on radii coming from all 

 parts of the circumference. It can in this way detect 

 vibrations in all directions, as it is in tactile com- 

 munication with every area of the snare. It is 

 interesting to see the spider testing the radii at 

 every thrill and feeling their tension when it is 

 doubtful of its capture. 



I have thought that the stimulus to produce that 

 wonderful accuracy in the reversal of the spiral might 

 be the differences in tension along the unequal radii 

 of the eccentric snare. For since a spider is able to 

 estimate changes in tension, it is probable that it could 

 also differentiate between the long and short radii of 

 an eccentric snare, which would certainly, if they were 

 wires or strings, give to the human fingers very per- 

 ceptible differences in sensation. By the difference in 

 tension the spider should discriminate between a radius 

 on the broad side and a radius on the narrow side of 

 the centre. At the moment of attachment of the spiral 

 to a radius the spider probably estimates the length of 

 free radius between the point of attachment and the 

 centre of the snare ; and in an eccentric snare all these 

 lengths will vary. The sensation produced by contact 

 with long radii would be of a different nature to that 

 produced by the short radii, and the spider may react 

 to the former stimulus by a reverse. The outer por- 

 tions of the radii, to which the previous turns of the 

 spiral have already been attached, would not be con- 

 sidered by the spider, as that portion would be damped 

 by the turns of the spiral in the same way as the finger 

 damps off a segment of a violin string. 



