288 ARTHROPODS. 



another and another is formed all intersecting in one centre ; 

 in this way the radiating rays of the web are made. 

 Secondly, it starts from the centre, and moves from ray to 

 to ray in a long close spiral gradually outwards, leaving a 

 strong spiral thread as it goes. Thirdly, the spider moves 

 in a closer spiral from the circumference inwards, biting 

 away the former spiral, replacing it by another, which is 

 viscid and adhesive. It is to this that the web chiefly owes 

 its power of catching insects which light there. There is 

 usually a special thread running to the adjacent hole or nest, 

 and the entire fabric is wonderfully sensitive, for the spider 

 feels rather than sees when a victim is caught. Indeed a 

 little piece of cork, dexterously twirled on the end of a thin 

 wire, and made to strike the strands of the web, will often 

 deceive the cunning spider. 



The spun threads are used in many other ways. They 

 line the nest, and form cocoons for the eggs. They often 

 trail behind the spiders as they creep, for instance over a 

 ploughed field. They greatly assist locomotion, and are 

 used in wonderful feats of climbing. Small and young 

 spiders often stand on their head on the top of a fence, 

 secrete a parachute of threads, and allow themselves to be 

 borne by the wind. 



Courtship. — The males are usually much smaller than the 

 females. It is calculated that the disproportion is sometimes 

 such as would be observed if a man 6 feet high and 150 

 pounds in weight were to marry a giantess of 75-90 feet high, 

 200,000 pounds in weight. I believe that the smallness of 

 the males is in great part due to the fact that they are males ; 

 others explain it by saying that the smaller the males are the 

 less likely they are to be caught by their frequently ferocious 

 mates. It is difficult, however, to understand how this 

 characteristic smallness, though perhaps advantageous and 

 likely to be favoured by natural selection, could be entailed 

 on the male offspring only. But this difficulty in regard to 

 inheritance is one which besets many similar applications of 

 the theory of natural selection. 



The males are often more brilliantly coloured than the 

 females, mainly, I believe, because they are males, though 

 what the physiological connection between the male consti- 

 tution and bright colours in this case is we cannot tell till 



