THE ORDERS OF FLIES. 189 
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 (Tinez) and (Noctue), 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, moreover, 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 
