10 THE BOOK OF THE FLY 



In the entomological s\>tematist's classification 

 the primary separation of flies into two sub-divisions 

 starts with a difficulty, for it is based upon circum- 

 stance often obscure and in some cases at least 

 imperfectly known. 



The first sub-division, Diptera Orthorrhapha, com- 

 prises those flies which in the stage of the pupa or chry- 

 salid disclose the outline of the perfect insect; in the 

 other sub-division, Diptera Cyclorrkapha, there are 

 grouped together all those flies of which the larvae make 

 for themselves a puparium or barrel-like case out of 

 their larval skin. 



The first mentioned sub-division comprises all the 

 gnats, midges, and most of the slender flies which are 

 outside the scope of the present work, but it also 

 includes a few kinds of more stoutly built flies, to 

 which some allusion will be made in the following 

 pages, as for example, the breeze-flies, Tabanidce. 



The second sub-division comprises many families, 

 including the muscid-like flies, of which the house-fly 

 is the type. The flies of this type are to be found in 

 the families of Muscida, Anthomyida, Tachinida t and 

 Cordylurida, comprising nearly 700 British species, of 

 which many rather closely resemble one another when 

 superficially observed. 



The approved classification of flies is to some extent 

 dependent upon the formation of the antennae, but the 

 unique feature of the systematic differentiation is based 

 upon a very intricate method of scrutinising, identify- 

 ing, and numbering the vein-like strengthening ribs 

 called veins, nervures, or nerve-lines, which, starting 

 from the shoulder, mark with characteristic patterns 



