BY R. J. TILLYARD. 



873 



will be dealt with fully later on in this paper. The normal 

 tracheation is shown in Text fig.l, the aberration in Text-fig. 4. 



Having thus satisfied ourselves that Neosticta is in line with 

 all other Zygoptera in possessing an unbranched radius and a 

 Zygopterid sector, Ms, arising from M or one of its branches, 

 let us now review the whole question at issue. This is a very 

 fundamental one, and may be put as follows : — 



Is the Zygopterid sector, Ms, homologoics with the Radial Sector, 

 h's, of Anisoptera, or merely analogous with it ? 



In a previous paper (3), I contended that the two were not 

 homolosous. No rebuttal of the evidence there adduced has 

 appeared in print; but I have received numerous ciiticisms. 



Text-fig. 1. 

 Normal tracheation of larval wing of Xeosficta caiiewens Tillyard. 



ranging from a complete acceptance of my views (Dr. F. Ris), 

 down to an absolute unwillingness to agree with them, in spite 

 of the evidence I have selected Dr. P. P. Calvert's criticism 

 as a very fair statement of the case in favour of the homology 

 between Rs and Ms. He writes as follows: — 



"If Ml, M.,, Mg, and M^ are homologous in Zygoptera and 

 Anisoptera, as you say on p. 224 (3), is there not something more 

 than the analogy between the Anisopteran Rs and the Zygopteran 

 Ms which you recognise on the same page"? If Zygoptera and 

 Anisoptera had a common origin, must not Ms and Rs have 

 come from one and the same primitive trachea 1 If this be the 

 case, then the distinction between Ms and Rs is really not such 

 a fundamental one after all. If, on the other hand, Anisoptera 

 and Zygoptera had not a common ancestry, then the apparent 



66 



