ENGINEERING SKILL OF SPIDERS. 



211 



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in addition the spider began to run up and down 

 her snare, the j^ebble was simply lifted up by the 

 tightening of the upper lines of the snare. This 

 result was i)robably assisted by the natural con- 

 traction of the elastic threads, and by the pulling 

 of the shrubbery under the force of the wind. 

 My conclusion was, therefore, that the spider had 

 balanced her 

 orb in the 

 usual way, 

 but discover- 

 ed that, al- 

 though it was 

 "founded up- 

 on a rock," 

 her house was 

 rendered in- 

 secure by the 

 simple fact 

 that the rock- 

 was not able 

 to keep its 

 place against 

 the strain. 



Mrs. Eigenmann sent me from Southern California some 

 cocoons of Zilla x-notata, from which I succeeded in raising 

 a number of broodlings, who domiciled in my 

 library. Many tiny snares were woven upon a 

 potted ivy plant, and one of these gave an ex- 

 ample of ready adaptation. The stay lines of 

 the orb were attached to sundry leaves, and the 

 upper and side attachments proved secure. But the leaf 

 to which the whole lower system of supports was fastened 

 fell off and stretched them downward, giving the snare a 

 peculiar, elongated shape, which I have never seen quite 

 approached. (Fig. 202.) This swinging pedestal amply 

 served the purpose of the wee architect, whose frail web 

 (draAvii here natural size) was well balanced even by so 

 light a weight. The weaving went on upon this new 

 basis, radii were spun down into the elongation, and 

 when the spiral loops were put in, that part was not 

 omitted. Thus, the net space available for business was 

 a good deal enlarged, and what was lost by the free 

 sector at the top was quite made up by the netteil 



Fig. 202. Sectoral orb of Zilla counterpoised by a fallen ivy leaf. 



su 





Zilla's 

 Leaf 

 Counter 

 poise. 



