Ill] THE WINGS 49 



suggests that they were originally separated by a convex supple- 

 ment. Such a supplement occurs in the archaic Petalura, starting 

 about eight cells distad from the sub-nodus (fig. 20). In the 

 Zygoptera, the corresponding "post-nodal sector," M 1A , is convex. 



Phylogenetic Studies. 



Comparison between the Wings of Anisoptera and Zygoptera. 



The original ancestor of the Winged Insects had, no doubt, 

 the meso- and metathorax separate and of equal size. Con- 

 sequently, the fore and hind-wings may be conceived of as having 

 been originally equal. They were developed as broad lateral 

 expansions or "planing areas," used at first simply on the para- 

 chute method. Later on, they took on the active function of 

 flight, and became true wings. Owing to the limited extent of 

 each thoracic segment, the fore-wings in the oldest insects in- 

 variably partly overlapped the hind-wings. In insects of strong 

 flight, this is clearly a disadvantage. Hence it came about that 

 the fore-wing, very early in the course of its phylogeny, tended 

 to become somewhat narrower than the hind, i.e. an anisopterous. 

 condition was set up. The earliest ancestors of the Dragonflies 

 known to us the Protodonata of the Upper Carboniferous 

 exhibit such a moderate anisopterous condition (fig. 156). The 

 essential difference between our two living suborders, the Anisop- 

 tera and the Zygoptera, is that, in the former, the tendency has 

 been to increase the anisopterous condition (for the culmination of 

 the evolutionary effort, see the wings of Tramea, fig. 136) ; while, 

 in the latter, owing to the reduction of all the wings alike into 

 mere "sculling organs," with no power of soaring or "planing," 

 the original anisoptery has tended to disappear, both fore and 

 hind-wings becoming almost equally narrow, as in the Agrionidae. 

 In nearly all cases, however, the evidence of original anisoptery 

 is preserved in the fact that the hind-wing is a little shorter, a little 

 broader, and very slightly less petioled than the fore- wing, as careful 

 measurements of almost any species will shew. 



Another fundamental difference between the two suborders is 

 found in the condition of the two largest wing-veins R and M. 

 If we turn again to the Protodonata (p. 304), we find that the 



