22 A VENATIONAL STUDY OF THE ZYGOPTERA 



all cases to separate the two, the general wing proportion and ar- 

 rangement of the veins do. In Platycnemis (fig. 74) and its allies 

 the wing is long and with the longitudinal veins arranged in a very 

 regular and parallel fashion. The cells are largely square or rec- 

 tangular, while in the group containing Coenagrion (fig. 94) there is 

 a tendency for M1+2 to bend posteriorly at the subnodal region and 

 for M4 and Cu. to be zigzag so that many cells are pentagonal. 

 The methods of specialization in the two groups are also different, 

 in the Platycnemis s'eries are retained some of the features to be 

 found in the Megapodagrioninae; for example, (i) great petiola- 

 tion which may extend beyond the level of the arculus, and (2) an 

 outward migration of the base of Rs to a considerable distance be- 

 yond the subnodus. In the second group, on the other hand, petio- 

 lation is delayed for a long time, so that it never reaches the level 

 of the arculus, and Rs arises beyond the subnodus in Amphicncmis 

 (fig. 129) only. Many of the Coenagrion group are further special- 

 ized by a great reduction in the number of cross veins in the wing. 

 To come now to the discussion of the legion Platycnemis in de- 

 tail, it is perhaps to be noted first of all that this may not be a 

 natural group, it may represent a collection of forms each of which 

 is the result of the dropping out of sectors from some corresponding 

 Megapodagrionine. If Megapodagrion (fig. 57) itself, by way of 

 example, were to lose its supplementary sectors, the resulting vena- 

 tion-would be much like that of Platycnemis (fig. 74) or Calicnemis 

 (fig. 72). The same might be true'of Wahnesia (fig. 65) and Tatoc- 

 nemis (fig. 78), of Heteragrion perhaps, (fig. 68) and of Prionocne- 

 mis (fig. 79). However this may be, and although these genera 

 resembling Platycnemis may not have come from a common an- 

 cestor, they do form a remarkably close and well connected series 

 beginning with Metacnemis (fig. 71), Calicnemis (fig. 72), and 

 Platycnemis (fig. 74), in which the wing is not petioled to the le\el of 

 Ac, and R, begins at the subnodus. Proceeding through Idiocnemis 

 (fig. 75), Coeliccia (fig. 76), and Allocuemis (fig. 77), petiolation and 

 the outward movement of the base of R. achance until the two 

 tendencies reach their culmination in Talocucmis (fig. 78) and 

 Prionocnemis (fig. 79) respectiv'ely. 



