Structure and Economy of Spiders. 479 



sorts proceed from the same instrument, is truly wonderful." 

 The fact, however, is even more extraordinary than it is repre- 

 sented to be by those distinguished naturalists ; for not only the 

 garden spider, but every geometric species with which I am 

 acquainted, employs three distinct kinds of silk, if a liquid gum 

 can with propriety be termed silk, in the construction of its net. 

 The boundary lines, radii, and first formed spiral line being 

 unadhesive, and possessing only a moderate share of elasticity, 

 are evidently composed of a different material from the last 

 formed spiral line, which is exceedingly viscid, and elastic in a 

 remarkable degree. Now, the viscidity of the elastic spiral 

 line may be shown to depend entirely upon the globules with 

 which it is studded ; for if they be removed by careful applica- 

 tions of the finger, a fine glossy line remains, which is highly 

 elastic, but perfectly unadhesive. As the globules, therefore, 

 and the line on which they are disposed differ so essentially 

 from each other, and from the rest of the snare, it is reasonable 

 to infer that the physical constitution of these several portions 

 of the net must be dissimilar. 



When exposed to the desiccating influence of the sun, and of 

 air briskly agitated, the nets of geometric spiders speedily lose 

 their adhesive property ; but when formed in situations from 

 which light is excluded, and where the atmosphere is not liable 

 to be perceptibly disturbed, I have known them retain their 

 viscidity for a long period. In a net of Epeira Diadema con- 

 structed in a glass jar, which was placed in a dark closet where 

 the temperature was not subject to great or sudden fluctuations, 

 the globules preserved their adhesive power almost unimpaired, 

 and the last-formed spiral line its elasticity for more than seven 

 months. 



The belief that spiders are incapable of ascending the per- 

 pendicular surfaces of polished bodies without the assistance of 



VOL. XVI. 3 Q lines 



