320 MONOGRAPH OP THE GENUS SYNTHEMIS, 



Petalura, we see that instead of an anal loop there are nearly 

 straight veins descending from various points of the anal vein 

 itself right to the posterior border of the wing. These can still 

 be traced(see fig. 10) in several species of Synthemis, and they 

 form the sides of the anal loops. Such straight veins are, in one 

 way, lines of weakness in a wing, since tliey make the posterior 

 margin very liable to tear. How quickly, for instance, does the 

 hind-margin of a Gomphine wing wear out ! The advantage of 

 the loop is not without many parallels in Nature. A very telling 

 one, it seems to me, can be seen in the difference between the 

 straight veins of the leaves of a monocotyledonous plant and the 

 " archings " of the secondary and tertiary veins in those of a 

 dicotyledonous plant. In the latter no vein that sets out towards 

 the leaf-border reaches it, but arches over on to the next vein; 

 between two arches, another smaller arch will be formed, and so 

 on; the final result being that all border veins run roughly 

 parallel to the margin. Such a leaf is not easily torn or split, 

 whereas a straight-veined leaf splits with great ease. The 

 pressure on the anal area of a dragonfly's wing during flight is 

 probably as great as that of a strong wind on a leaf, and hence 

 the development of loops should be expected, especially in those 

 dragonflies which have a long seasonal range or indulge in long 

 continuous flights. Petalura, though of great size, never indulges 

 in long flights. The Gompldniti have a very short seasonal range 

 and are seldom on the wing. The necessity, therefore, for the 

 development of loops in these insects has scarcely arisen. But 

 in Synthemis, where the seasonal range is quite as long as in most 

 dragonflies (many species that emerge in November last quite to 

 the end of March, and S. guttata may be taken right into May), 

 and when the insects are continually in flight whenever the sun 

 is out, the need for wing-strengthening has been much greater. 

 It points, therefore, to very feeble specialisation, that the group 

 has not yet developed a strong and invariable loop, such as is 

 found in Macromia. 



Pterostigma. — A strong braced vein on the edge of tlie stigma 

 is a sign of high specialisation, and may be seen in the most 



