THE ORDERS OF FLIES. 1 89 



The second order of pricking flies, namely, the Two- 

 winyed Flies (Diptera), are also found in a fossil state 

 in the oolites, together with Half-winged Flies ; but they 

 probably developed out of the Hemiptera by the degenera- 

 tion of the hind wings. In Diptera the fore wings alone 

 have remained perfect. The principal portion of this order 

 consists of the elongated gnats (Nemocera) and of the compact 

 blow-flies and house-flies (Brachycera), the former of which 

 are probably the older of the two. However, remains of 

 both are found in the oolitic period. The two small groups 

 of lice-flies (Pupipara) forming chrysales, and the hopping- 

 fleas (Aphaniptera), probably developed out of the Diptera 

 by degeneration resulting from parasitism. 



The eighth and last order of Flies, and at the same 

 time the only one with mouth-parts adapted to sipping 

 liquids, consists of moths and butterflies (Lepidoptera). 

 This order appears, in several morphological respects, to 

 be the most perfect class of Flies, and accordingly was 

 the last to develop. For we only know of fossil remains of 

 this order from the tertiary period, whereas the three 

 preceding orders extend back to the oolites, and the four 

 biting orders even to the coal period. The close relation- 

 ship between some moths (Tinese) and (Noetuse), and some 

 caddis-flies (Phryganida) renders it probable that butterflies 

 have developed from this group, that is, out of the order of 

 Gauze-winged Flies, or Neuroptera. 



The whole history of Flies, and, moreovei-, the history 

 of the whole- tribe of Arthropoda, essentially confirms 

 the great laws of differentiation and perfecting which, 

 according to Darwin's theory of selection, must be 



