BY R. J. TILLYARD. 875 



haps, at first evident. Actually, they assume a position of 

 immense importance, and must be fully dealt with if the problem 

 is to be solved. 



I propose now to take these three kinds of evidence in full, 

 and to draw definite conclusions from them. 



1. The Structural Evidence. (Text-figs. 1, 3). 



We may best marshall this by gathering together all the 

 results of wing-tracheational studies by different students of the 

 Order, and arranging them in tabular form. I have before me 

 the published results of Professor Needhani(l) and myself (2, 3), 

 together with the beautiful series of photographs taken by Dr. 

 Ris from the larvae of lAhellula, Calopteryx (Z, Plate xxxiv.), and 

 Ischiiura. No doubt other students have examined othergenera; 

 but, as their statements have not been published, they cannot 

 be here included. 



The point to notice is that the evidence is, ivifhout a sinyle 

 exception, in favour of the view that trachea Rs in Anisoptera is 

 not homoloyous with trachea 2/s in Zyyopiera. It follows also 

 that, if the veins Rs and Ms in the imaginal wing are laid down 

 in their entirety upon the previously existing trachete Rs and 

 Ms respectively, then vei7i Jis in Anisoptera is not homoloyous 

 with vein Ms in Zyyoptera. This second conclusion must not, 

 however, be accepted without a fuller examination of the inter- 

 play of tracheational and venational developmental forces, which 

 are discussed later on, on p. 879. 



