BY R. J. TILLYARD. 



G59 



Text-Fig.85. 



Tracheation of forewing of freshly formed pupa of Xyleutes eucalypti 

 (Scott), (x 3-3). Lettering- as on p. 535. 



some months. Of the former I obtained and dissected several 

 pupae, with very interesting results. 



Text-fig. 85 shows the tracheation of the forewing of a pupa 

 only a day or two old. The resemblance of this wing to that 

 of the Hepialidae is very striking, and is the more noteworthy 

 when we consider how very different the imaginal forewings of 

 these two families appear. Comparing Xyleutes with Leto 

 (Text-fig. 80), we note that the tendency towards the splitting- 

 back of the tracheae is much more marked in the latter (and in 

 all Hepialidae), and that the former is the more archaic in 

 having trachea Cu 2 of full length. The subcostal veinlet is 

 supplied, in both forms, by a special trachea arising alongside 

 Sc. In Xyleutes there is a well-marked series of branch- 

 tracheae arising from Sc and proceeding towards the costal 

 border. Similar tracheae can be seen in the pupae of many large 

 Lepidoptera, but in none are they so well developed as in this 

 family. I have figured them here in order to call attention to 

 the obvious fact that, if veins were to be formed over the courses 

 of these tracheae, there would result the original condition of a 

 complete set of costal veinlets, such as has been preserved in 

 the Megaloptera and Planipennia at the present day. As long 

 as these tracheae still exist in the pupae of Lepidoptera, can 

 anyone deny the possibility of the evolution of a new line, in 



