460 



28. — More use has been made of the wings and wing venation in 

 following out genealogical development than of any other single por- 

 tion of the body of the dragon-fly. The evidence is conflicting in 

 many respects, and in coming to conclusions all characters must be 

 taken into account. The most noticeable feature of the wing venation 

 is the crossing of the longitudinal veins Rs and M. This condition 

 is so unique that it was doubted or denied for a long time, and not 

 until it was traced from its beginning in the tracheae of the nymph 

 was it generally accepted as true. Many of the changes in the wing 

 venation may be considered as the result of stress on particular por- 

 tions of the wing surface. The development has followed two lines 

 of specialization; one of them a reductive process, exemplified in the 

 Zygoptera, the other additive, exemplified in the acquisition of im- 

 portant wing-braces in the wings of Anisoptera. 



The main points regarding the specialization of the odonate wing 

 are stated in the following tabulation. 



Generalised conditions 



1. Wings of equal size and 



venation. 



2. Wings not petiolate. 



3. Nodus not retracted ; 



near the middle. 



4. No reduction in number 



of cross-veins. 



5. Arculus near the base of 



the wing. 



6. No reduction in the 



number of antenodals. 



7. No reduction in the 



number of postnodals. 



8. Rs 



9- 



10. 



traceable throughout 



its course. 

 M2 not arising distad of 



the nodus. 

 Rs separating from M2 



near the nodus. 



Developmental tendencies 



Ti o 

 ° O 



(/} a, 



Wings of unequal size and Yes Yes 



venation. 

 Wings petiolate. Yes Yes 



Retraction of the nodus to- Yes Yes 



wards the base. 

 General reduction in number Yes ? 



of cross-veins. 

 Retreat of the arculus distad Yes Yes 



from base. 

 Reduction in number of an- Yes ? 



tenodal cross-veins. 

 Reduction in number of Yes ? 



postnodal cross-veins. 

 Rs not traceable throughout No Yes 



its course. 

 Mo arising distad of the Yes ? 



nodus. 

 Rs separating from Mg dis- Yes ? 



tad of the nodus. 



