BRITISH BLOODSUCKERS 253 



silently up and settle on the most exposed parts of 

 the legs and flanks. They are very voracious, and 

 manage to devour an amount of blood which is 

 truly surprising. 



A little examination of the gadfly will show you, 

 too, one important point in which it and all other 

 true flies differ from the bees, wasps, butterflies, 

 and the vast mass of ordinary insects. All the 

 other races have four wings, and I showed you in 

 the case of the wasp the beautiful mechanism of 

 hooks and grooves by which the fore and hind 

 wings are often locked together in one great group, 

 so as to insure uniformity and fixity in flying. 

 Among the true flies, however, including not only 

 the house-fly and the meat-fly, but also the gadflies 

 and the mosquitoes, only one pair of wings the 

 front pair is ever developed. The second or hind 

 pair is feebly represented by a couple of tiny rudi- 

 mentary wings, known as poisers or balancers, 

 which you can just make out in the sketch, like a 

 couple of stalked knobs, in the space between the 

 true wings and the tail or abdomen. It is pretty 

 clear that the common ancestor of all these two- 

 winged flies must have had four wings, like the 

 rest of the great class to which he belonged ; but 

 he found it in some way more convenient for his 

 purpose to get rid of one pair, and he has handed 

 down that singular modification of structure to all 

 his descendants. Yet whenever an organ or set 

 of organs is suppressed in this way, it almost always 

 happens that rudiments or relics of the suppressed 

 part remain to the latest generations ; and thus the 



