GEOMETRICAL SPIDERS 91 



diverge from the hub of a wheel. We ask ourselves, 

 how is this symmetry attained ? By what mechanism 

 does the spider measure with such accuracy the same 

 distance between each pair of radii ? Let us watch 

 the spider at work. It has completed the foundation- 

 lines, and is now throwing out its radii. Backwards 

 and forwards from the centre to the circumference we 

 see it hasten. Out along one spoke, back along 

 another spoke, and on each return journey a new 

 radius is secure. For a moment it halts at the centre. 

 Something engages its attention here. It busies itself 

 about the hub. It rotates from side to side. It 

 is examining the radii all round the snare, satisfying 

 itself that in one part they are complete, that in 

 another part they have not yet been spun. We watch 

 with care this examination of the radii. We see that 

 the spider with the tips of its fore legs is feeling 

 and testing the radii just at the point where they leave 

 the hub. We see a pair of legs expand like a 

 mathematical dividers ; the tip of one leg rests on one 

 radius, the tip of the other leg on the adjoining radius ; 

 and it is clear that the spicier is measuring the inter- 

 radial distance by using its legs as a pair of dividers 

 while it remains seated at the hub. The limbs are 

 kept at an even distance and the spider stretches 

 forward to feel all round the snare. Should it feel 

 a radius with the tip of each limb, then it knows that 

 the radii are complete in that segment and are at the 

 correct interval. It then turns to another part of 

 the snare and again feels for the radii. It has now to 

 expand its legs more widely to feel two adjoining 

 radii, and it therefore knows that here the radii are 

 incomplete, Legs expanded the normal width mean 



