32 



A VENATIONAL STUDY OF THE ZYGOPTERA 



various members of the Megapodagrioninae shows it is often ap- 

 parent only; but even if so a combination of the preceding ten- 

 dency and of this one indicates specialization. 



A study of almost any group will show the same thing; one that 

 is a good example is the Platycnemis group of the Coenagrioninae 

 (see Table IV). • 



Table IV 



The Ratio to the Length of the Wing of the Distance from the 

 Base of the Wing to the Subnodus and Points of Origin of M2, Rs, 

 and M3. 



A first glance at the wings themselves might suggest to one that 

 M2, Rs, and M3 arise further out in the wing in the last genera than 

 in the first, but an inspection of the above table shows that in 

 general this is not true. It is not even equally true of the different 

 veins but may be more so of some than others; Rg for example in 

 Prionocnemis (fig. 79) does arise beyond what it does in the others, 

 M2 and M3 really arise nearer the base in the more specialized than 

 in the lower wings. The subnodus becomes more retracted, quite 

 consistently so, and the space between it and the points of origin of 

 M2 and Rs enlarges, while the relative positions of the subnodus 

 and the base of M3 change. 



A comparison of the Agrionidae and of the Coenagrionidae shows 

 on the whole, that in the lower subfamilies of the latter family the 

 vein Mi+2 forks at about one-half the wing-length, as it does in the 

 former family, but in the higher subfamilies the movement out- 

 ward is evident. In the whole series Rg and M3 also migrate out- 

 ward. Dr. Calvert says that "the paleontological evidence is not 

 altogether satisfactory and at least admits of the possibility of the 

 separation — points of these three veins having fluctuated proximad 

 or distad from time to time." 



