148 ANIMAL LIFE IN DESERTS 



appearance by being clothed with hairs of that 

 colour ; many of these insects settle rarely or never 

 on the desert soil, so that their case deserves special 

 consideration. The phenomenon is found in many 

 bees, wasps, and fossors, and in at least three 

 famihes of flies (Bombyliidse, Nemestrinidae, and 

 Tabanidae). That is to say that it has been evolved 

 on a number of separate occasions to suit the particu- 

 lar needs of insects that fly or hover over deserts. 

 Morice was, I beheve, the first to call attention to it 

 in Hymenoptera, remarking that " no one, I should 

 think, who has collected in the deserts of Algeria, 

 Egypt or Palestine can doubt that the very pale 

 colours and shimmering silvery or golden pruinosity, 

 so common in aU groups of Hymenoptera in such 

 locahties, and practically there only, are cryptic 

 characters, rendering the insects which possess 

 them inconspicuous and almost invisible among the 

 guttering sand and pale vegetation which they 

 haunt. Here at least, contrary to the rule laid down 

 by Wallace, we have stinging Hymenoptera coloured 

 so as to resemble, or at any rate so as not to con- 

 trast with, the inanimate and vegetable substances 

 which normally surround them." I have quoted 

 his words because they describe the facts excel- 

 lently ; his suggestion that the colours are " cryp- 

 tic " is a topic to which we shall return later. A 

 similar condition prevails in the gad-flies (Tabanidae), 

 many species of which from Mesopotamia and 

 South Palestine are clothed with down of a pale 

 tawny colour. This is remarkable, because the gad- 

 flies rarely settle on the ground or on rocks except 



