588 



THE PANORPOID COMPLEX, 111., 



one genus, Perissectis, of which a large supply was available, it 

 was found that the majority of wings showed M4 present. 

 Moreover, specimens were found, in all the genera examined, in 

 which all stages of the reduction of this trachea were to be 

 seen. A few specimens were also found in which trachea M 4 

 persisted at the expense of trachea Cu la) the latter finally be- 

 coming aborted. The condition in which both trachea? were 



Text-Fig.53. 



Condition of the tracheation in the imaginal wings of Hepialidae , in the 

 region of the distal Y-vein. a, both trachea M4 and trachea Cui a 

 complete, in Perissectis australasiae Don; b, trachea Cui a becoming 

 reduced, in Charagia spleiidens Scott; c, further reduction, in 

 Pielus sp. ; d, complete loss of trachea Cui a , in Charagia astatlies 

 Turn. Lettering as on p. 535. 



well developed is shown in Text-fig. 58, a, while the stages in 

 the reduction of trachea Cu la are illustrated in Text-figs. 58, b-d. 

 An examination of the imaginal venation shows that, in a 

 number of specimens, the chitinisation of the vein M4 is stronger 

 than that of Cu la though the reverse is usually the case. This 

 suggests that in the earliest Lepidoptera, vein M 4 was well 

 developed, but was tending to become weakened by its fusion 



