PHILIP A. MUNZ 37 



of the Anisozygoptera and among the possible ancestors of the 

 Zygoptera. 



The other method of bracing is for the inner end of the stigma 

 to be pointed and form a Y, the base of which is made by that por- 

 tion of Ri situated just before it. In this way Ri holds the stigma 

 firmly. Both methods may occur in the same wing. The Poly- 

 thorinae have this Y well developed. 



3. Atrophy of the stigma. — The diffusion and even total disap- 

 pearance of the stigma in genera of two subfamilies, the Agrioninae 

 and Pseudostigmatinae, may not be considered by some as a sign 

 of advancement, but since the Odonata as far back as the Mesozoic 

 do have the stigma, and since the modern genera which stand at the 

 base of the Agrionine and Pseudostigmatine lines have it, and its 

 gradual disappearance is traceable in a series, its atrophy must 

 be taken as a matter of specialization, if not necessarily one of 

 advantage. 



IV— SUMMARY 



From the preceding discussion it is evident that venation gives 

 In most cases an easy and fairly clear means of determining rela- 

 tionships, but in certain ones it may be capable of more than one- 

 method of interpretation, and must therefore be checked up by 

 other characters if a final decision is to be reached. Sometimes 

 this doubtful condition is due to parallelism, sometimes to contra- 

 dictory evidence from several sources; one character in the wing 

 may indicate one tendency, another quite a different one. 



The keynote to the phylogeny of the Zygoptera as based on 

 venation is reduction. With this in mind one can divide the sub- 

 order into two families, the lower of which, the Agrionidae, has as 

 lateral offshoots: (i) the Polythorinae (figs. 2-5) in which M4 does 

 not descend the arculus and the quadrangle is therefore broad at 

 the base and concave above;' (2) the Agrioninae (figs. 6-20), where 

 the wing fails to become petiolated, but the stigma atrophies and 

 the quadrangle becomes somewhat convex above and widened 

 distally; and (3) at a much higher level, the Disparocyphinae (fig. 

 37) as a direct ofiEshoot of the main line of descent, by a shortening 

 of Cu2, until it is represented by a cross-vein only. This main line 

 of descent is to be found in the Epallaginae (figs. 21-36) which 



MEM. AM. ENT. SOC, 3. 



