XXIII INSECTA : DIPTERA 359 



Another species forms a hairy tubular gall standing on the 

 upper side of the leaf of ground-ivy ; another, a white pimple- 

 like hairy gall on the under side of the 

 leaf of meadow-sweet ; another causes a 

 swelling on the woody twig of the 

 sallow willow ; and there are, besides 

 these, many other gall-forming species. 



Sub-order 3. Aphaniptera or Fleas. 



Fleas are sometimes considered as 

 peculiarly modified Diptera, though Fio. 286.— Gall ("a") on 

 they are also sometimes placed in a J-^«--^ ^^ ^-- 

 special order because of the many 

 points in which they differ from Diptera and from any other 

 order of insects. 



They are always wingless, and are very compressed later- 

 ally. The head is relatively small, and the first pair of legs 

 is turned forward in such a way that they appear as if 

 growing from the head. The mouth-parts are peculiar, the 

 labial palps forming sheaths which enclose three needle-like 

 pricking organs, recalling the lancets of the gnat. The eggs 

 of a flea are white and oval. They usually drop to the ground, 

 and the tiny, white, worm-like larvae thrive in any little 

 accumulation of dirt, feeding on it. They make, when full 

 grown, little cocoons covered with dust, inside of which the 

 larvae pupate, the perfect fleas emerging in a week or two, 

 and then feeding as parasites on the blood of some animal host. 



The flea which troubles man is Pvlex irritans, but there 

 are many other fleas peculiar to other mammals or to birds, 

 and these will often transfer their attentions from one kind 

 of animal host to another. 



Classification of the Diptera mentioned in Chapters 

 XXII. and XXIIL 



Order. Diptera. 



Sub-order 1. Brachycera. 

 Family 1. Muscidae. 



Genera. Musca (House-fly). 



Galliphora (Blue-bottle). 

 Glosdna (Tsetse-fly). 



