114 A NATURALIST IN HIMALAYA 



mechanical motion of the spider round and round the 

 web and to ponder over the heedless instinct that 

 compels the same eternal round. But still more 

 strange would be the picture of the snare if instinct 

 was so blind as to allow of no reversal and to compel 

 the spider in the same headlong course. The whole 

 fabric would be unsymmetrical, all its perfect beauty 

 would be lost. The astonishment is not in seeing 

 the monotonous routine of instinct, but rather in 

 seeing that instinct so plastic as to enable the spider 

 to achieve this perfect symmetry no matter how great 

 is the eccentricity of its snare. 



Is it possible to gain any clue as to what guides the 

 spider in the performance of this essential act ? How 

 does the spider know when to reverse and when to 

 pursue an unbroken course ? I cannot with complete 

 confidence explain this, but I strongly suspect that it 

 is guided by its fine sense of touch and the power to 

 estimate the tension of its lines. I have observed 

 that a small species of Epeira, which constructs its 

 snare in the coniferous forests, seems distinctly to 

 possess the power of discriminating between any 

 alterations of tension that may exist at different parts 

 of the snare. This spider weaves its web at the point 

 of radiation of three foundation-lines. If one of these 

 foundation-lines be gently stretched away from the 

 snare, the spider, while resting at the centre, will 

 immediately recognize the change in tension that has 

 occurred. It will form a correct impression of the 

 direction of the abnormal strain, and will advance 

 to investigate the cause of the disturbance. I am 

 satisfied that the Epeira can discriminate between 

 different states of tension, and we should remember 



