422 THE STUDY OF INSECTS. 



four or to three. Vein V is three-branched in the more generalized 

 forms. Vein VII is two-branched. Vein VIII is usually merely a 

 concave fold just behind vein VII and parallel with that vein ; it is 

 represented in most of the figures of wings by a dotted line. Vein IX 

 is usually present; and sometimes vein XI also exists. 



One of the most marked features in the specialization of the wings 

 of Diptera is a tendency of the veins to coalesce from the margin of 

 the wing towards the base. This is illustrated by the wing of Conops 

 (Fig. 496). In this genus veins IIL+s and V J+2 coalesce at the mar- 

 gin of the wing; veins V 3 and VI Ij coalesce for nearly their entire 



v, 



vn, 



VII, 



FIG. 497. Wing- of Rhyfhus. 



length. The result of this coalescence is to cause the free part of 

 vein V 3 to appear like a cross-vein between cells V and the ist cell 

 V. Veins VII 2 and IX also coalesce at the margin of the wing. 



In a few genera of flies certain longitudinal veins are bent so as 

 to form a sharp angle, and from this angle a spur is developed. Thus 

 in Protoplasa there is a sharp angle near the base of vein III 2+ 6 which 

 bears a spur (Fig. 504, s) ; in Erax a similar spur is formed on vein 

 III 4 (Fig. 559, s) ; and in Pantarbes this spur on vein III 4 is prolonged 

 so as to form a complete cross-vein dividing cell Ills into two parts 

 (Fig. 564). 



v 



TABLE FOR DETERMINING THE FAMILIES OF THE DIPTERA. 



A. Flies in which the abdomen is distinctly segmented, and the two 

 legs of each thoracic segment are not widely separated. Habits 

 various, but the adults do not live parasitically upon either birds 

 or mammals. 



B. Antennae consisting of more than three segments. (Note that 

 a style or bristle borne by the third segment is not counted as a 

 segment.) 



