462 NEW AND RARE AUSTRALIAN AGRIONID^, 



A. exsudans with those of Ayrion JyelJi, we cannot fail to 

 notice a similarity in the build of all three. On the other 

 hand, the appendages of .4. hi/acin finis suggest rather those 

 of Ischnura heterosticfd and /. torresiana ; while those of A. 

 riihricauda and Ausirocnemis splendida have some slight 

 resemblance (in profile) to those of Caliayrou biiliiiyhursti. 

 These resemblances, together with the proved fact that the 

 Australian species of the genus Ayrinrnemis are larger (less 

 reduced) than their close allies from Java and Ceylon, seem 

 to point us to the following conclusions. The southern repre- 

 sentatives of what was, originally, a large bipolar concern 

 (viz. Ayrion, Isrhinini, and allied genera) had to face some 

 intense competition which their northern congeners did not 

 meet with. (Probably this was the invasion of their original 

 sphere by the Australian Lestes-growp). Ousted from this 

 position, the whole southern branch was driven northwards 

 towards the Tropics, and rapid asthenogenesis set in as it 

 fought its way along. A few species held their own (Ayrion 

 lyelli, Ischnura heterosticta) \ a large number of the more 

 robust forms became reduced to the Pseudayrion-iorm ; and 

 some of these, proceeding even further north, degraded to 

 Stenobasis and Telehasis. The weaker species, still refuging, 

 as far as possible, in their old regions, but driven to astheno- 

 genetic excess by the competition of their more successful 

 rivals, became rapidly reduced to the extreme stages shown 

 in Ayriocnemis and Hem.iphlehia. The few of these that 

 worked through Malaysia would, of course, show a still fur- 

 ther diminution in size along the most northerly limits of 

 their range. This hypotheses explains not only the types of 

 appendage found in Ayriocnemis, but also the absence of a 

 large group of Ayrion and Ischnura in the Southern Hemi- 

 sphere. Tt also explains how it is that Ayriocnemis and allies, 

 though driven to extreme asthenogenetic limits, can still 

 exhibit an inferior sector of the triangle as well developed 

 as in Ayrion itself. What the larger species gained by the 

 reduction of this vein, the smaller gained by extreme reduc- 



