PHILIP A. MUNZ 23 



Leptocnemis (fig. 79) and Paracnemis which are rather typical 

 forms of the Platycnemis group, except in the proportions of the 

 quadrangle, and Antiagriofi (fig. 81) a segregate of the old genus 

 Coenagrion itself, offer themselves as transitional genera to the 

 remaining Coenagrioninae. 



These other genera in turn, can be divided into two groups: the 

 Argia group and that of Coenagrion proper. In the former the 

 inner end of the quadrangle is generally longer than the upper part 

 of the arculus and as long as the upper side of the quadrangle, which 

 is widened distally. Another character, not venational, is the 

 presence of long tibial spines. This group has its own evolutionary 

 line which begins with Hyponeura (fig. 82) and Palaiargia (fig. 83) 

 and proceeds by greater petiolation, shortening of Cu2, and reduc- 

 tion of the number of postnodals through Argia (fig. 84) to Argial- 

 lagma (fig. 86) and Diargia (fig. 85). 



• Of the Coenagrion group proper, it may be said that venation 

 offers fewer marks for identification than in any other group of the 

 Zygoptera, and it is here that keys for determination must be 

 artificial. The genera are so numerous and so much alike that 

 other characters are necessary for their differentiation. Venation 

 does, however, indicate the general tendencies of phylogeny. Mr. 

 Tillyard (191 2) may be quoted as follows, "Those forms are the 

 least asthenogenetic in which the basal postcostal nervule* is closest 

 to the base of the wing, the petiolation consequently least marked, 

 and the inferior sector of the triangle longest and best developed. 

 Such forms, while marking the 'high-water mark' of success- 

 ful coenogenetic specialisation in the legion, must nevertheless 

 be considered to be more in line with the ancestral stem of the 

 group than are their asthenogenetic off-shoots." Following this 

 theory, one can readily arrange a series which begins with Oreagrion 

 (fig. ^j) and Pyrrhosoma (fig. 89) and, by a shortening of Cu2 and a 

 reduction in the number of postnodals, continues through EnaUagma 

 (fig. 98), Ischnura (fig. 100) and Coenagrion (fig. 94) to another 

 dividing point, one branch leading off in the direction of reduction 

 in size and further decrease in the number of postnodals until 

 Ceratura (fig. 102), Anomalagrion (fig. 97), Agriocnemis (fig. 104) 



* Basal postcostal nervule is the vein Ac of this paper. Inferior sector of the tri- 

 angle is Cu2. 



MEM. AM. ENT. SOC, 3. 



