BY R. J. TILLYARD. 885 



discussion and table, may be bi-iefly contrasted witli the older 

 (Needham's) view as follows : — 



{ I) Need ham's vie?/; .• —When the fusion or crossing of two 

 main tracheae causes the formation of a unilateral fork, pre- 

 ceded by a weak area not possessing a main trachea, this weak 

 area and the fork are secondarily strengthened, in the imaginal 

 venation only, by the backward development of a strong vein 

 called a bridge. The base of the unilateral fork persists as the 

 oblique vein. 



(2) 7'he netv view : — When the fusion or crossing of two main 

 trachene causes the formation of a unilateral fork, preceded by a 

 weak area not possessing a main trachea, the imaginal venation 

 does not follow this specialisation in the tracheal system, but 

 adheres to the older and more advantageous venational plan. 

 Thus, the imaginal vein originally present fersists as a hridye 

 along that part of the wing now devoid of a main trachea. The 

 point of attachment of the shifted trachea is represented in the 

 venation by the oblique vein. 



The principal argument in favour of the new view seems to 

 me to be this:— If we take any specialised Odonate wing, in 

 which an oblique vein is present, and replace the oblique vein 

 by an ordinary cross-vein, then the venation will be seen to 

 resemble what is admitted to be the most archaic venational 

 plan for the sub-order. For instance, if w^e cut out the oblique 

 vein from Lestes or Synlestes, the arrangement of R, M and 

 branches at once becomes similar to that of the archaic Meya- 

 podagj-ioiiirue, in which no oblique vein or bridge has ever been 

 developed. We may well ask, how is it that there is a bridge 

 developed in this position in every form where tracheational 

 specialisation has set in 1 If Needham's view be correct, then 

 there must have been a period in the evolution of the wing-vena- 

 tion (before the bridge became fully formed) in which the place 

 of the bridge was taken by the irregular boundaries of polygonal 

 cells from which the bridge is supposed to have arisen. Can 

 anyone maintain that such a weakened imaginal form would 

 have been able to hold its own during the evolution of the 

 bridge? Or, if it is argued that the bridge was formed link by 



