178 DEVELOPMENT OF THE WING-VENATION OF ODONATA, 



cells, is formed on its distal side by a strong descending branch 

 of Cug, which we call Cuot,. 



It is evident from this, that the elongation of the anal loop 

 was first brought about by the inclusi(jn of extra sets of cells 

 beyond the original distal end still to be seen in Stage 2. If, in 

 the loop of Synthemis {Fig.7), we imagine the wing to be some- 

 what stretched in a slantwise direction so as to straighten out 

 Cuj, Aj at the same time becoming straighter but not longer, 

 while A., becomes both straighter and also considerably longer, 

 we get an approach to Stage 3, which can then be completed by 

 a basal narrowing and by the development of a strong Cugb dis- 

 tally. Hence we see that the loop of Austrocordulia is a com- 

 posite structure, and no longer a true " anal loop," i.e., no longer 

 enclosed purely between Aj and A.,. 



Further peculiarities in this area of the wing of Austrocordulia 

 must here be noticed. 'N'ow that A^ has obtained the command- 

 ing position under Ac, A3 has shifted considerably basad, and 

 become reduced in size. Hence arises the beginning of the 

 decline of the aiial triangle in the male — a movement which leads 

 inevitably to the complete loss of the angulated wing, as seen in 

 Heniicordidia and Lihellulinff . Hence, in so far as the elonga- 

 tion of A 2 necessitates the reduction of A3, so far may we say 

 that the Italian loop is developed at the expense of the anal 

 triangle. In Tetragoneuria (Needham, loc. cit., Fig. 19, p. 721) 

 we see the maximum development permissible to both at the 

 same time; a development w^iich may well be claimed to be the 

 high- water mark of Anisopterous evolution, and from which the 

 whole mass of the Lihelluliiue may be judged to stand on a lower 

 level by regression, as they certainly do in powers of flight. 



In Austrocordulia, the hind wing-triangle is greatly stretched 

 out longitudinally, more so than in any other Libellu/id, yet its 

 basal side does not quite succeed in reaching the arculus. This 

 excessive stretching, together with the extreme narrowing of the 

 proximal part of the loop, are special to this genus, and are not 

 to be judged as developments along the direct line between Stages 

 2 and 4. 



