30 



A VENATIONAL STUDY OF THE ZYGOPTERA 



amount of space back of the anal bridge and therefore the possi- 

 biHty of cross-veins occurring there. 



Vein Shifting 

 I. Narrowing of the wing. — This tendency is coupled with the 

 first one mentioned in the previous group, namely the reduction in 

 the number of sectors. When the sectors drop out, the principal 

 veins come to lie closer together. The wing may become narrower 

 also by the veins Cui and Cuo becoming reduced. A comparison 

 of the typical genera of the subfamilies shows at a glance how preva- 

 lent this tendency is (see Table II). 



Table II 



The Ratio of the Width of the Wing to the Length in the Dif- 

 ferent Subfamilies. 



It is evident in this table, what effect the dropping out of Cuo in 

 Disparocypha and Protoneura has on the width of the wing. In 

 Lestoidea, however, the presence of two sectors between Mi and 

 Mia, has offset this tendency (see fig. 45). 



The Pseudostigmatinae is a good example of the same tendency 

 (see Table III). The subfamilies Megapodagrioninae, Coena- 

 grioninae, and Epallaginae might be used for similar tables. 



An examination of Handlirsch's figures in "Die Fossilen In- 

 secten" show some rather narrow wnngs in periods as remote as the 

 Mesozoic; evidently narrowing of the wing and loss of the sectors 

 are ancient tendencies. 



