SPIDEES— WEB-BUILDING. 209 



^ous structures should never have been developed in the 

 case of any other animal having predaceous habits — 

 especially, perhaps, in that of the imago form of preda- 

 ceous insects. It is easy to see how, if there were any 

 original tendency to secrete a viscid substance in the 

 .neighbourhood of the anus, this might be utilised in de- 

 scending from low elevations (as certain kinds of slugs use 

 their viscid slime as threads whereby to let themselves 

 down from low branches to the ground) ; and so we can 

 understand how natural selection might thus have the 

 material supplied out of which to develop such highly 

 specialised organs as the spinnerets of a spider. But if 

 we are inclined to wonder why this should not have 

 happened among other animals, we must remember that 

 any expectation that it should rests on negative grounds ; 

 •we have no reason to suppose that in any other case the 

 initial tendency to secrete a viscid substance was present. 

 One inference, however, in the case of spiders seems per- 

 fectly vahd. As this comparatively rare faculty of web- 

 spinning occurs so generally throughout the class, it must 

 have had its earliest origin very far back in the history of 

 that class, though probably not so far back as to include 

 the common progenitors of the spiders and the scorpions, 

 seeing that the latter do not spin webs. 



I shall now give a few details on the manner in which 

 spiders' webs are made. Without going into the ana- 

 tomy of the subject further than to observe that a 

 spider's ' thread ' is a composite structure made up of a 

 number of finer threads, which leave their respective 

 spinneret-holes in an almost fluid condition, and immedi- 

 ately harden by exposure to the air, I shall begin at once 

 to describe the method of construction. 



The so-called ' geometric spider ' constructs her web 

 by first laying down the radiating and unadhesive rays, 

 and then, beginning from the centre, spins a spiral line of 

 unadhesive web, like that of the rays which it intersects. 

 This line, in being woven through the radii in a spiral 

 from centre to xiircumference, serves as a scaffolding for the 

 spider to walk over, and also keeps the rays properly 

 stretched. She next spins another spiral line, but this 



