THE WINGS 



61 



rectangle. Owing to the fact that the sectors of the arculus 

 arise lower down in the latter than in the former subfamily, 

 the quadrilateral of the latter is usually excessively narrow. 

 In the Thorinae (fig. 21) the sectors of the arculus are actually 

 attached to R. This position has been strengthened, in Thore, by 

 a thickening of the antenodal just above it. The effect is to 

 produce a quadrilateral of peculiar and, indeed, unique form, with 

 the basal side more than twice as long as the distal. 



Fig. 21. 



Base of fore-wing of Thore gigantea Selys. Ax thickened antenodal; 

 Ms Zygopterid sector. Adapted from Needham. 



We have seen that, if the level of the arculus be very close to 

 the level of the bifurcation of Cu, the quadrilateral will be very 

 short. Such a condition also suggests the possibility of a new 

 arrangement, whereby the arculus, by assuming a sufficient slant, 

 might be continued directly on to the point of division of Cu. In 

 this case the lower portion of the arculus would not be needed as 

 a support, and so the discoidal cell would remain open basally. 

 The Jurassic subfamily Tarsophlebiinae (fig. 158) adopted this line 

 of development. Quite recently, as the result of extreme reduction, 



Fig. 22. Wings of Hemiphlebia mirabili* Selys, ^ (hind-wing 11 mm.). Original. 



