374 



THE WINGS OF HYMENOPTERA 



Reduction by coalescence. — An excellent illustration of the reduction in 

 number of distinct wing-veins by the coalescence of adjacent veins is to be 

 seen in the fore wing of an ichneumon-fly (Fig. 393), where the thickened 

 margin of the wing, between its base and the stigma, is formed of the united 

 costa, subcosta, radius, and media. A similar condition exists in the fore 

 wing of a braconid (Fig. 394). 



In many of the Clistogastra in which the costa remains separate, the 

 subcosta, radius, and media ccalesce from the base of the wing to the 

 stigma. 



Modification of the course of a vein by coalescence at base. — Perhaps 

 the best illustration of the modification of the course of a vein by the 



Fig. 395. — Wings of Manoxyela (From MacGillivray). 



coalescence of its base with an adjacent vein is that of the media of the fore 

 wing; for in this case a very complete record of the change exists among 

 living Hymenoptera. Correlated with the modification of the course of 

 media are certain changes in the relations of the medio-cubital cross-vein; 

 hence the two veins will be treated together. 



In Manoxyela, as was shown by Dr. MacGillivray (Fig. 395), the main 

 stem of the media retains a cjuite primitive course except that the basal part 

 coalesces with the radius for a considerable distance; for from the point 

 where it separates from the radius it extends directly toward the outer 

 margin of the wing. In this genus, the medio-cubital cross-vein also retains 

 a comparatively primitive position (Fig. 395, m-cu). 



In Macroxyela (Fig. 392), the medio-cubital cross-vein has migrated 

 towards the base of the wing, the coalescence of the media with the radius 

 has been extended somewhat, and the section of the media between the 

 radius and the medio-cubital cross-vein has become nearU^ transverse. 



