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er principal veins. This stage is exhibited by Prionoxystus, 

 (Fig. 12), in which radius of the hind wings still persists but is 

 much weaker than the other veins, except media. Correlated 

 with this weakening of radius is the formation of a cross vein 

 between it and subcosta (Fig. 12, c. v.^. This is an estab- 

 lishment of a new source of air supply for the distal portion of 

 radius, and renders less necessary the basal portion of that 

 vein. Sometimes the two veins are drawn together, and the 



Fig. 'ii^.—Protoparce. 



coalescence extends in both directions from the point of first 

 union, resulting in the form presented by Megalopyge (Fig. 

 25) ; but in other cases that portion of radius between the 

 point of union and the base of the wing becomes atrophied. 

 An approach to this condition is shown by Acoloithis (Fig. 

 26). 



In the fore wings where radius is branched, we often find 

 an anastomosing of the branches. In this way are formed 

 the accessory cells (Fig. 20). This anastomosing doubtless 

 serves to strengthen the wing. 



