BY R. J. TILLYARD, 209 



wing. Thus it develops into a four-branched trachea supplying 

 a greater area of wing-rudiment than is supplied by any other 

 trachea. 



It might here be accepted as sufficient explanation of the 

 ultimate fusion of R and IM in the imago, that it was the direct 

 outcome of the struggle for precedence between these two leading- 

 tracheae of the wing. Certainly this played no inconsiderable 

 part in the result. But there can be little doubt that another 

 factor, viz , the gradual tendency towards the production of a 

 narrower flying-wing from an originally broader planing-area^ 

 helped to drive these two tracheae close together. The same 

 tendency also most certainly played a part in the invasion of 

 territory originally served by R, by one or more branches of M. 

 Partly because of this gradual wing-narrowing, partly also 

 because of the gain in development by M at the expense of R, 

 we find the former throwing two of its four branches over Rs, so 

 as to invade and supply the area between R^ and Rs. 



(5) The cubitus(Cu) was originally a much smaller trachea than 

 either M or R. In a wing with a symmetrical oxygen-supply 

 {i.e., a supply received equally from costal and anal ends of AT), 

 Cu would develop about equally with Sc, its analogue on the 

 costal side of the wing. But it lies in a more favourable position, 

 in the Odonate wing, since it is closer to the anal oxygen-supply 

 than any trachea except A itself. Hence it develops into a 

 strong two-branched ^-rachea of greater importance than any 

 except R and M. 



The weaker development of Cu (and A also) in the forewing is 

 easily explained by the fact that the hind wing-case completely 

 covers over this portion of the forewing. Thus, both Cu and A 

 in the forewing are deprived of the light so necessary to the 

 formation of strong pigmentation; and this must, in the end, 

 have a deleterious effect on the development of this portion of 

 the forewing. 



(6) The anal trachea (A) is only a small trachea, which could 

 develop but little^ were it not for its extremely favourable posi- 

 tion, enabling it to take first toll on the available oxygen-supply, 



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