GEOMETRICAL SPIDERS 101 



Just as a human being, when drawing one Hne 

 parallel to another line, uses the position of the first 

 line to judge with his eye the accuracy of the second 

 line, so does the spider in weaving the beautiful 

 parallel texture of its snare use the position of one 

 turn of its spiral from which to measure with its fore 

 leg the accurate position of the next turn. 



Observation of the spider at its work makes us 

 almost confident of the truth of this. But its move- 

 ments are so agile ; the turn of its body, the gentle 

 touch with the fore leg, the rapid application of the 

 spinnerets, follow one another so quickly that it is 

 difficult to be certain of the sequence of events. If 

 the fore leg is indeed the vital organ in the operation, 

 the measuring rule by which one line is laid parallel 

 to the preceding line, then a few careful experiments 

 should confirm the truth of the belief. 



If the spider places one turn in position by estimat- 

 ing the distance from the preceding turn, then if I 

 divide the preceding turn, the spider will have lost its 

 point of measurement and parallelism will be destroyed. 

 I choose one segment for the experiment when the 

 spider is working at its viscid spiral. It has just 

 crossed the segment, leaving a filament in its train. 

 I divide the filament. The spider circles on, laying 

 down the spiral to perfection until it again reaches the 

 experimental segment. Arriving at this segment, what 

 happens ? The spider as usual stretches forth its leg 

 to feel for the line laid down in its last circle. But the 

 line is gone. The spider finds nothing there. It 

 stretches still further forward and feels the next line, 

 the one laid down in its second last circle. This line 

 it takes to be the true line. From it the measure- 



