186 A BOOK OF INSECTS 



of " aggressive mimicry," as it is called, is unsupported 

 by direct evidence ; nor is it easy to believe that these 

 swift and agile flies stand in need of any such aid. It is 

 therefore more reasonable to suppose that the mimicry is 

 really protective, serving to exempt the robbers themselves 

 from the attacks of larger insectivorous creatures. 



None of the Asilidce has yet acquired the habit of 

 feeding upon warm-blooded animals ; but many other Dip- 

 tera obtain their food in this way, although it is a remark- 

 able fact that (with few exceptions) the females alone are 

 blood-suckers. The males, if they feed at all, are vege- 

 tarians ; while it is known that many, if not all, of the 

 females are capable of subsisting upon the juices of plants 

 if they are debarred from their favourite food. These 

 considerations suggest that blood-sucking (except among 

 the truly parasitic Diptera) may be a newly acquired 

 habit, due to the enterprise of the so-called gentler sex. 

 These matrons seem to have discovered that the vital 

 fluid of mammals is more easily assimilated, and more 

 sustaining, than the raw sap of plants ; and there is reason 

 for thinking that the bad habit may spread in course of 

 time not only to their mates, but to other species which 

 are at present outside the secret. 



In a few instances blood-sucking is already indulged 

 in by both sexes. This is the case with the African 

 tse-tse flies, and with three British representatives of the 

 great family Muscidce. Among the latter, the best known 

 is the so-called biting house-fly, or stable-fly (Stomoxys 

 calcitrant). It has a close superficial resemblance to its 

 namesake, but it carries a mouthful of lancets, and although 

 it may often be seen imbibing nectar from flowers, it is 

 an inveterate blood-sucker when occasion offers. Like 

 the house-fly, Stomoxys breeds preferably in stable refuse. 

 But it is seldom found in houses except just before rain, 



