226 WING-VE??ATION IN ZYGOPTEItOUS DRAGONFLIES, 



some distance from the base, and there is a wide anal wing- 

 space left between it and the base without tracheal supply. In 

 the imago, it seems to be represented by the first cubito-anal 

 cross-vein, Ac being the third, which is often seen to be slightly 

 oblique (Fig.4). 



C. Supplementary Bridges in Calopterygidcp.. 



In Biphlebia, and probably also in Philogcmga, the long sup- 

 plement, lying between Mj and Mg, is fed by a trachea which 

 drops from Mj, and supplies weak distal and proximal branches 

 to the developing supplement. In the imago, this condition is 

 reproduced by an oblique vein, representing the stem of the 

 descending trachea, and a short "bridge," which is, in reality, 

 only the proximal portion of the supplement. Anyone who will 

 examine the formation of Mspl and Rspl in Anisoptera, particu- 

 larly in jEschnince, will see a similar method of tracheal supply 

 carried to excess. In jEschna brevistyla, I have sometimes seen 

 as many as three depending tracheae, all supplying the supple- 

 ment. As, however, they are all of more than one cell's length, 

 they do not get mistaken for "oblique veins" in the imago. But 

 they are homologous with the latter, and the proximal parts of 

 the supplements are the homologues of "bridge-veins." 



A similar short " bridge-formation " is observable (apparently) 

 extending Ms very slightly proximad in Chalcopteryx and Rhino- 

 cypha. This simply shows us that, in the Calopterygidce, the 

 wing required extra support in the region just distal from the 

 arculus. In some forms, Ms did not arise sufficiently close to 

 Mg to give the necessary support, so a short " bridge " was 

 developed to supply the deficiency. In others (Calopterygince), 

 extra support was gained by a fusion with the radius. 



Thns we see that short " bridges" and "oblique veins" may be 

 developed in connection with almost any sector or supplement 

 which requires strengthening basally. The "bridge" of Ani- 

 soptera and the "long bridge" of Lestidce are the only two, out of 

 probably very many attempts of this kind, which have attained 

 real systematic importance, by attaining a definite structure 

 and constancy throughout a whole group of genera. 



