52 



THE WINGS 



[CH. 



border, close to the tip, at the point of greatest impact with the 

 air during flight, was clearly a necessity. The close approximation 

 of R to C just at this point in the Protodonata provided the re- 

 quired means, viz. a thickening of the membrane in the narrow 

 space between them. Thus arose the most primitive type of 



B 



C 



Fig. 18. Various forms of pterostigma. A. Ancestral (Protodonate) condition. 

 B. Petalura ingentissima Tillyard. c. Acschna brevistyla Ramb. D. Hemi- 

 gomphus heteroclitus Selys. B. Nannophya dalei Tillyard. F. Macromia 

 terpsichore Forster. G. Diphlebia lestoides Selys. H. Austrolestes analis 

 Ramb. j. Ischnura heterosticta Burm., <$, hind-wing. K. The same, fore- 

 wing. L. Anomalagrion hastatum Say. (after Needham). M. Mecistogaster 

 Iwretia Drury, $, fore- wing. N. Calopteryx splendens Harris, $. o. The 

 same, <J. (B-D, o, H x 3; B, F, J, K, M x6; sr, o x 8.) Original, except L. 



Odonate pterostigma, the exceedingly long, narrow, and strongly- 

 braced form still preserved to us in the Petalurinae (B). Such 

 a form is, indeed, very little removed from a simple fusion of C 

 and R, such as is seen in some Protodonata. 



A decrease in length and an increase in thickness brought about 

 the next stage, such as we find in the Aeschninae (c). In many 



