BY R. J. TILLYARD. 219 



It was in the hopes that the CalopterygidcE would show us 

 some clue to the phylogenetic develoyment of Rs, that I took up 

 the work embodied in this paper. It seemed certain that they 

 alone held the key. So, indeed, it has turned out, but how 

 different (and how much simpler, after all !) is the solution they 

 give us from the one we expected. I feel that I cannot present 

 the case better than by quoting from Dr. Ris's letter, accom- 

 panying the photographs of Calopteryx larval-wings. And I 

 must here add, that to Dr. Ris undoubtedly belongs the credit of 

 first seriously entertaining the new ideas, which were slowly 

 forcing themselves upon us, but which both he and I, for so long 

 a time, remained unwilling to accept. He says: — "Now what 

 do you think of this troublesome matter of zygopterous venation, 

 with those ontogenetic documents in hand % I said to myself : 

 'Supposing you did not know a word of anisopterous venation, 

 of Rs crossing over branches of M, of bridges, oblique veins, and 

 the rest — how would you interpret this nymphal tracheation "? 

 Well, I think the obvious interpretation would be — Vein R in 

 this wing is unhranched] all the sectors are branches of the media 

 M ! With comparatively little modification (shifting of second 

 and third branches of M — numbered from base — distad) of this 

 scheme, you arrive at what is shown in the Agrionidce. And 

 still, with not very great modification, you arrive at Lestes and 

 Synlestes, as shown by Needham's and your own photos.; shift 

 the second branch still further distad, until it becomes a tribu- 

 tary of the third one, and let it be united (for obvious mechanical 

 reasons !) to the main branch by a bridge — and you will have the 

 Lestine type of venation. All this is so very simple (and ruinous 

 to my favourite construction of November last!) that I do refrain 

 from accepting it as given truth. Can there be anywhere in 

 development a shifting of an existing radial sector over to the 

 media, and the traces of this fact be so utterly lost as it appears 

 from the examined wings ? Now please think of that matter, 

 and let me know again ! " 



In a later letter he replied to some criticisms of mine : " You 

 know, I am hesitating in interpretation of Zygopterous veins; 

 old views are shaken; new ones not yet fully established; my 



