38 A VENATIONAL STUDY OF THE ZYGOPTERA 



advance by petiolation, by reduction of the number of cross-veins, 

 especially of the antenodals, and by a retraction of the nodus 

 toward the wing-base. The Epallaginae lead to the second family, 

 the Coenagrionidae, with which they are connected by various inter- 

 mediate forms: Diphlebia (fig. 34), Philoganga (fig. 36), Thauma- 

 toneura (fig. 51), Ortholestes (fig. 39) and Pentaphlehia. Early in 

 the Coenagrionidae the nodus becomes more retracted, the number 

 of antenodals becomes established as two, and the number of sup- 

 plementary sectors shows a decided tendency to be decreased. 

 There are two general lines of descent as regards the points of 

 origin of Rg and M3; in the first one these veins retain the condition 

 of the Agrionidae and arise nearer the arculus than the subnodus; 

 this is true of the Lestinae (fig. 38-44) and Lestoidinae (fig. 45). 

 In the Lestinae is developed a quadrangle which is acute distally, 

 and in the other subfamily the vein Cu2 is again reduced so that it 

 does not extend beyond MA. The other line in the Coenagrionidae 

 has Rs and M3 migrate outward so that they arise nearer the sub- 

 nodus than the arculus. The Pseudostigmatinae (figs. 46-50) are 

 an immediate offshoot, with greatly elongated wings in which the 

 nodus is retracted and the stigma becomes diffuse. The Megapo- 

 dagrioninae (figs. 51-70) form the central branch which leads to 

 the Coenagrioninae (figs. 71-129) by the loss of all sectors except 

 Mia, and also continues in its own direction by greater petiolation 

 and outward progression of the base of M3. The Coenagrioninae 

 are also characterized by a slight movement outward of M3 and by a 

 great reduction in the number of postnodals; petiolation on the 

 whole does not proceed very far. This family in turn gives rise to 

 the Protoneurinae (figs. 130-153) by the reduction of Cuo, a change 

 which has thus taken place in three groups of Zygoptera. In the 

 Protoneurinae Cui also becomes short, and in the highest form, 

 Selysioneura (fig. 153), Cu does not branch at all. 



V— KEYS FOR THE DETERMINATION OF GENERA 



For the most part these keys indicate as nearly as they can be 

 made to do so, the same views regarding the phylogeny of the 

 Zygoptera as have been expressed in the preceding pages. The 

 Coenagrioninae have been the stumbling block for such a system, 



