FOOD OF INSECTS. 419 



pairs, serve for a considerable period ; but the nets of 

 the geometric spiders are in favourable weather renewed 

 either wholly, or at least their concentric circles, every 

 twenty-four hours, even when not apparently injured. 

 This difference in the operations of the two tribes de- 

 pends upon a very remarkable peculiarity in the con- 

 formation of their snares. The threads of the house 

 spider's web are all of the same kind of silk, and flies 

 are caught in them from their claws becoming entangled 

 in the fine meshes which form the texture. On the other 

 hand the net of the garden spider is composed of two 

 distinct kinds of silk ; that of the radii not adhesive, that 

 of the circles extremely viscid 3 . The cause of this dif- 

 ference, which, when it is considered that both sorts of 

 silk proceed from the same instrument, is truly wonder- 

 ful, may be readily perceived. If you examine a newly 

 formed net with a microscope, you will find that the 

 threads composing the outline and the radii are simple, 

 those of the circles closely studded with minute dew-like 

 globules, which from the elasticity of the thread are easily 

 separable from each other. That these are in fact glo- 

 bules, of viscid gum, is proved by their adhering to the 

 finger and retaining dust thrown upon the net, while the 

 unadhesive radii and exterior threads remain unsoiled. 

 It is these gummed threads alone which retain the in- 

 sects that fly into the net; and as they lose their" viscid 

 properties by the action of the air, it is necessary that 

 they should be frequently renewed. 



a May not the spinners mentioned by Leeuvvenhoek (see above 

 p. 407, note) be peculiar to the retiary spiders, and furnish this vis- 

 cid thread ? 



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