in] THE WINGS 51 



To sum up, then, we may indicate the basic differences between 

 the two suborders as follows : 



Anisoptera Zygoptera 



1. Tendency to accentuate the original Tendency to obliterate the same 



anisopterj 



2. Basic condition of S once-branched, of Basic condition of R vnbranched, of 



M three-branched JJ Jour branched 



3. Discoidal cell divided into two Discoidal r-ell a single complete whole 



We may add that, on the totality of evidence, there is no 

 justification for assuming that one suborder is more archaic than 

 the other. Recent writers have invariably assumed that the 

 Zygoptera are more archaic than the Anisoptera. The truth 

 seems rather to be that the Anisoptera have preserved the ancient 

 strongly-built Protodonate type more completely, but have added 

 to it the larger number of caenogenetic developments. On the 

 other hand, the Zygoptera have undoubtedly departed far more 

 from the original "natural conception'' of a Dragonfly: but, 

 chiefly owing to the fact that their line of evolution has been 

 a gradual reduction of the basic structural plan, they have not 

 added definite new structures to the same extent as the Anisoptera 

 have. Take away its triangles, and Petahira undoubtedly repre- 

 sents the most archaic of all living Odonata, and the nearest 

 approach to Meganeura in size, strength, density of venation, and 

 general body-design. Ignore its obvious reduction to a weak type. 

 and Pseudophaea may lay claim to be equally as archaic as Petalura. 

 Again, the wonderful advance made by the Eucorduliini or Tra- 

 meini over the Petalura-type stamps these forms as the most 

 highly organized of all Dragonflies. But the advance made by 

 the Agrionidae over the Pseudophaea-type is every bit as great, 

 though in another direction; and the structure of their wings 

 probably represents, on the whole, an even wider departure from 

 the original type (taking function as well as structure into account) 

 than in the case of the Trameini. 



The Pterostigwa (fig. 18). 



A pterostigma occurs in other Orders of Insects besides Odoiiata, 

 but in none is it so constant or so conspicuous. From an evolu- 

 tionary point of view, a strengthening of the wing on the costal 



