100 A NATURALIST IN HIMALAYA 



it reached the next turn of the temporary spiral before it 

 could cross over. This did not alter the accuracy of the 

 spider's work. Without any hesitation it made use of 

 the inner bridge, and I was surprised to see that this 

 did not in any way interfere with the parallelism of 

 the viscid spiral which it was laying down. If, as I 

 imagined, the spider measured each turn of the viscid 

 spiral from the last turn of the temporary spiral, then 

 in that segment where the turn was divided the 

 parallelism should have been lost, for the point of 

 measurement was gone. But this was not the case. 

 The threads were as parallel in the experimental 

 segment as in any other segment. We must look to 

 some other mode of measurement to explain the secret 

 of the work. 



It is clear that the measurement is not made from 

 within. Perhaps it is made from without. The spider 

 may measure the turn of its viscid spiral from the turn 

 that lies just external to it and which was laid down in 

 its last circuit. We watch the operation with the 

 greatest care. Out along the radius moves the spider 

 until it reaches the point where it will attach its thread. 

 A peculiar motion now occurs and this is the secret of 

 its work. The attachment is not yet made. The 

 whole body of the spider first undergoes a partial 

 rotation ; the fore limb is extended forward ; the 

 sensitive tip gently touches the filament of the viscid 

 spiral laid down in the previous circuit, and, when this 

 is done, the spinnerets are applied to the radius and 

 the attachment is made. What is the spider doing ? 

 Surely it is using its fore limb to measure the correct 

 distance from the previous turn of the spiral at which 

 to make its new attachment. 



