FLIES: TWO^WINGED INSECTS. 



DIPTERA 



Members of this order are Mosquitoes, Gnats, and 

 Flies. The last name is applied, with modifying adjec- 

 tives, to many other insects, but true flies never have more 

 than one pair of wings. The pair of small, knobbed or- 

 gans, called balancers or halteres, just back of these 

 wings, represents a second pair of wings. The scale-like 

 affairs above the halteres and back of the roots of the 

 wings are called squama or calyptrae (some authors call 

 them tegulas or alulae) ; there may be two pairs, one pair, 

 or none. Eggs of Diptera are sometimes called "nits"; 

 the larvae are called "maggots," "wrigglers," or "bots." 

 Pupation often occurs inside the larval skin. About 

 10,000 species are already described from North America. 



The venation of the wings and the arrangement of the 

 thoracic bristles are important in classification; also the 

 antennas, which vary greatly from group to group. Un- 

 fortunately there are several systems of names for the 

 veins and cells of the wings but the following (see Plate 

 LXIX) is in rather general use. The vein which forms 

 the front margin of, and runs for a variable distance 

 around, the wing is called the costal (or marginal}. The 

 next vein back of it is the auxiliary; then come the longi- 

 tudinals (first to fifth), the last three of which are often 

 branched. The cell between the costal and auxiliary 

 veins is the costal cell; that between the auxiliary and first 

 longitudinal veins is the subcostal cell; then, in order along 

 the margin of the wing, are the marginal, first submarginal, 

 second submarginal (between branches of the third longi- 

 tudinal vein, if branched), and the more or less numerous 

 posterior cells. The central cell is the discal; and at the 

 base, from front to back, are the first basal, second basal, 

 anal (not always present), and axillary cells. A cell is 

 said to be complete when it is entirely enclosed by veins. 



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