BY R. J. TILLYARD. 119 



a complete success, which is here reproduced in Text-fig. 1. As 

 the venation was complete, and even the delicate pigmentation of 

 the imaginal wing could be noted, I was easily able to determine 

 this wing as belonging to the rather rare insect Xantholeon 

 hdmsi Tillyard, — a result as fortunate as it was unexpected, since 

 Xantholeon is certainly one of the most archaic of our Australian 

 Myrnidpontid(e. 



The result of this examination can be gathered by comparing 

 Text-figs. 1 and 2. It will be seen that it not only bears out in 

 full my suspicions concerning the real position of Cuo, but shows 

 also a further unexpected point of interest. For the trachea 

 Cu2 is not a weak remnant descending to 1 A, as the examination 

 of the imaginal venation had led me to suspect, but is a strong 

 trachea which has captured the line of lA, in the same manner 

 that M2 has captured the line of Cui,,. In both cases, it is the 

 upper trachea that prevails, the lower that goes under. In both 

 cases, the crossing of the upper trachea on to the level of the 

 lower is marked, in the imaginal venation, by an oblique vei7i, 

 which represents, of course, the actual basal piece of the upper 

 vein in question. We must therefore distinguish in future 

 between the median oblique vein {Om) and the cubital oblique 

 vein (Oc). This latter is very clearly marked in Xantholeon 

 (Text-fig. 2), also in Glenoleon, Acanthaclisis, and other genera; 

 but it cannot be satisfactorily made out in our Australian species 

 of the genus Myrmeleo7i, which are more highly specialised. 



The process by which M2 and Cug have attained their present 

 positions may be described as a process of "trachea-capture." 

 The two tracheae run at first more or less parallel. As the wing- 

 becomes narrower, they come to lie side by side. The oxygen- 

 supply conveyed by both is now no longer fully needed; so that, 

 in course of time, the weaker (in this case the lower) trachea 

 becomes aborted, being reduced to a remnant impinging upon 

 the strong upper trachea from below. In the resulting imaginal 

 wing-venation, however, where the determining factor is not 

 oxygen-supply, but the wing-stresses brought about during for- 

 ward flight, the vein formed along the coui'se of the lower trachea 



