Vol. ].] 



Woodivorth . — Wing Veins of Insects. 



107 



There do not seem to be sufficient grounds for giving the cross 

 veins the value of branches, as has been done by Comstock 

 and Needham. 



PSOCID^. 



As far as the venation is concerned, the Psocidae are very 

 completely isolated. There is a striking superficial resem- 

 blance between the venation of this group and that of the 

 Hymenoptera. Only when one attempts to compare carefully 

 the two is the illusion dispelled. No other group approaches 

 the Hymenoptera so closely in the shape of the wings as the 

 Psocidse. It may be that the similarity results from the veins 

 adapting themselves to meet an identical mechanical condi- 

 tion due to the shape of the wings. 



The typical venation, such as seen in Psocus, is given in 

 Fig. 44, and the scheme of venation of both pairs of wings in 

 Fig. 45. From the diagram it will be seen that, excepting the 

 Embiidae, there is 



less diversity than 

 in any of the groups 

 of lower insects, 

 and in this respect 

 the Psocidae are 

 quite comparable 

 with the Hymen- 

 optera or other 

 higher groups. 



The homologiz- 

 ing of the veins in 

 Psocidae has been a matter of controversy. There have been 

 two questions: One, the comparison of the veins of the front 

 wing with those of the hind wing; the other, the comparison 

 of the venation of Psocid wings with other venations. The 

 former is often avoided by disregarding the hind wing entirely. 

 There are two principal theories regarding the homologies of 

 the front and hind wing. According to one theory the pri- 

 mary vein is nearly suppressed, as it appears to be in the hind 

 wing of Psocus (Fig. 44); according to the other, this vein is 

 the one running nearly to the tip parallel with the margin. A 

 comparative stvidy of the wings of other genera shows that the 

 latter is clearly the case. 



The other question is more complicated. Recent studies 



FIG. 44. Venation of Psocus. 



