210 DEVELOPMENT OF THE WING-VENATION OF ODONATA, 



and thus to develop much more freely than its analogue (C) on 

 the costal side of the wing. Hence, in the Odonate wing, it is 

 enabled to become a four-branched trachea of considerable im- 

 portance, particularly in the overlying hindwing. Also, it com- 

 petes with Cu in the same manner that M competes with R; and 

 hence arises the fusion of Cu and A already described in the 

 imaginal wing. Of course, as in the case of R and M, the 

 tendency towards narrowing also aids this result. 



This theory, then, founded on the facts now known concerning 

 the origin of the alar trunk, is sufficient to account for all the 

 peculiarities of the Odonate wing, provided we accept also the 

 theory, already advanced by Handlirsch,* of the development of 

 the present-day insect-wing from an original broad plmihig-area, 

 only useful for accomplishing long downward ^^ vols planes,'^ and 

 not for active flight. Probably nobody will now refuse to accept 

 Handlirsch's theory, since it appears to be the only possible ex- 

 planation of the development of any form of wing in Nature. It 

 is not, however, the object of this Section to discuss Handlirsch's 

 theory. We may safely claim it as a contributory cause to the 

 peculiar development of the Odonate wing-venation, while, at the 

 same time, indicating the anal" oxygen-supply as the primary 

 cause of all those peculiarities. 



Let us now go back a little further, and inquire what evidence 

 there is, either in Ontogeny or Palaeontology, for the belief that 

 the larvae of Odonata were not origiiudly but only secondarily 

 aquatic in their mode of life. The evidence for this seems to me 

 to be overwhelming, but there are some important points that 

 bear more closely on the question at issue. 



First of all, if the Odonata larvse took to fresh water from the 

 sea, without the intervention of a land-living period, we should 

 naturally expect to find them still breathing by those archaic 

 adaptations of segmental processes which, we have every reason 

 to believe, were employed by Trilohites and their nearest allies. 

 The fact that the larvse of both Anisoptera and Zygoptera exhibit 



*Fossilen Insekten, pp.1316 et seq., 1908. 



