26 A VENATIONAL STUDY OF THE ZYGOPTERA 



tree of the suborder, and should bear the name of the man who has 

 done more than any other worker to further our knowledge of the 

 Odonata, namely Baron de Selys-Longchamps. 



In contrast to the Coenagrioninae the Protoneurinae do not have 

 the reduction in the number of postnodals, they are as numerous in 

 Selysioneura (fig. 153) as in Disparoneura (fig. 132). 



Ill— TENDENCIES IN SPECIALIZATION 



These have been rather fully discussed in previous papers, par- 

 ticularly by Dr. Needham (1903a) for the order Odonata as a whole, 

 and by Dr. Calvert (1913) for the Zygoptera with special reference 

 to the Megapodagrioninae. It is my purpose here to review their 

 opinions and to add a few observations of my own. 



I think it must be apparent from the preceding discussion on the 

 phylogeny that the outstanding features in the development of the 

 Zygoptera are a constant reduction and simplification of venation. 

 At any rate this is true from the standpoint of comparative mor- 

 phology and to a certain degree it seems to be so paleontologically. 

 As might be expected this reduction is to some extent accompanied 

 by a decrease in the power of flight. Dr. Calvert says, " Many of 

 the ideas which have been expressed on the changes which have 

 taken place in the venation of the Odonate wings have been based 

 on the comparative morphology of living representatives of the 

 order or on mechanical advantage. It does not follow that these 

 foundations are trustworthy or that there has always been mechan- 

 ical improvement in the wings. Degeneration is just as probable 

 as progressive development. The actual course of phylogeny can- 

 not be deduced from these considerations or from the data of 

 morphology. It seems therefore desirable to attempt to ascertain 

 what paleontological evidence exists affording clues to the descent 

 of these insects, by tracing the modifications which the wings 

 exhibit from the Carboniferous period down to the present time." 

 This is what he has done and free use is here made of his 

 conclusions. 



It should be noted that the tendencies here discussed are not all 

 present in the group, nor all to be found even in the highest groups, 

 but they are those which impress one, when studying over all the 

 groups, as being indicative of advancement and departure from the 



