104 LONDON INSECTS 



The latter are microscopic insects which live during 

 part of their lives as parasites on Bees. As we have 

 plenty of Bees we have probably also plenty of the 

 insects which live upon them. But as without very 

 good eyes and very good glasses, and time and patience 

 to use them, we are not very likely to find any ' screw 

 wings,' it is not necessary to say much about them 

 here. Their chief point of interest is that, exactly 

 reversing the arrangements of the ' two-winged ' order, 

 their upper instead of their lower pair of wings are 

 shrivelled up and apparently useless. The last remain- 

 ing order the 'nerved winged' is of far more general 

 interest, and contains in the Dragon-flies examples of 

 the most perfect development of powers of flight known, 

 compared with which the wings of a bird are clumsy 

 contrivances. 



The bodies of men and other terrestrial animals are 

 comparatively solid masses. Weight is no disadvantage 

 to them. If anything, it is an advantage, as it helps 

 them to force their way through the obstacles . which 

 they meet in their comings and goings on the dull 

 earth, and so every available corner of the trunk is 

 packed as tightly as possible. 



But with creatures which are to have the power of 

 lifting themselves from the ground the case is different. 

 What they most require is lightness, and so the hollows 

 of the bones, which in men and quadrupeds are used 

 as bottles and casks for holding marrow and other 

 liquids which are wanted to oil the joints and keep 

 the machinery in working order, are in most birds 



