BY R. .T. Tirj.YARt). 879 



devdopmevf, wliich is mainly concerned, tliroiighoiit larval life, 

 with supplying each cell of the developing wing with sufficient 

 oxygen, and the force of venational develupmenl, which is con- 

 cerned only with the final production, at metamorphosis, of an 

 imaginal wing-plan best suited to withstand the strains and 

 stresses of flight. Both these forces, in the evolutionary scheme, 

 aim at perfection. But, as tliey are at any rate to some extent 

 antagonistic the final result must be a compromise. Tliis com- 

 promise is V)est seen, in tlie imaginal venation, in the formation 

 of bridges and oblique veins — structures which, we may well 

 believe, are not wanted at all to ensure perfection of flight, but 

 which have been forced upon the imaginal pattern owing to the 

 trend of tracheational development. On the one hand, the 

 tracheational scheme appears early in the developing larval 

 wing; one might almost imagine that the forces controlling it 

 must have had everything their own way, and that the imaginal 

 pattern, when first laid down in the wing of the last larval 

 instar, must be absolutely dependent upon it On the other 

 hand, in spite of the long start gained by the traclieational 

 forces, can we doubt that it is the imaginal wing-plan which 

 must be aimed at all along] For, if this be not attained suc- 

 cessfully, the type would be weak in flight, and would soon die 

 out. 



It is just here that we have to study intensely the problem 

 of bridges and oblique veins. Professor Needham's now famous 

 discovery and explanatitjn of the bridge-veins in Odonata is 

 well-known, but needs to be shortly summarised. Jn the 

 Anisoptera, tlie bridge is, according to him, a "brace evolved 

 out of the boundaries of ordinary cells," in order to "correct the 

 mechanical weakness of the unilateral fork" (1, p. 755), {i.e., the 

 fork represented in tlie imaginal venation by the point of de- 

 partuie of Rs from Mo at O). Text-flg.3 will explain Needhams 

 view of the formation of the bridge. 



In Needham's view, also, the bridge is a "trunk SHcondarily 

 developed to connect the radial sector with vein Mj_.j." (1, p. 711). 



I think that all students of Odonata, including myself, have, 

 up to the present time, accepted these statements without ques- 



