CHAPTER XIX. 
Order DlPTERA (Dip'te-ra). 
The Flies . 
The members of this order have only two wings; these 
are borne by the mesothorax. The metathorax is furnished 
with a pair of knobbed threads , the halteres . The mouth- 
parts are formed for sucking . The metamorphosis is com¬ 
plete. 
To the order Diptera belong all insects that are properly 
termed flies, and only these. The word “fly” forms a 
part of many compound names of insects of other orders, 
as butterfly, stone-fly, May-fly, and Chalcis-fly; but when 
used alone, it is correctly applied only to dipterous insects. 
To some flies other common names have been applied, as 
mosquito, gnat, and midge. 
The name Diptera is from two Greek words: dis, two, 
and pteron , a wing. It was suggested by the fact that the 
flies are distinguished by the possession of a single pair of 
wings; for no fly has more than two wings, and only a few 
are wingless. 
The wings of flies are thin, membranous, and usually 
either naked or clothed with microscopic hairs; but with 
mosquitoes the wings bear scales, and with the moth-like 
flies (Psychodidas) and some others the clothing of hairs is 
very conspicuous. The hind wings are represented by a 
pair of knobbed threads, the halteres (hal-te'res); these can 
be easily seen in a crane-fly (Fig. 482). The function of 
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