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 ; forif 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. 3Q lines 
