Dr Hankin, The soaring flight of dragon-flies 465 



VIII. Comparison of low and high speed flight. 



As above stated, low speed flight is dependent on the presence 

 of sunshine. In the early morning, before the sun has attained its 

 full strength, it is clearly favoured by the absence of wind. On 

 occasions when the air was uncomfortably hot and very dry and 

 when, therefore, the slightest movement of the air could be recog- 

 nised by its cooling effect on the skin and when the dragon-flies 

 were gliding often within two or three feet of my head, it has been 

 noticed that the coming of the slightest draught of wind caused 

 gliding to be replaced by flapping. 



Sometimes during the daytime, if the air is nearly calm, wind 

 seems to be unwelcome to the dragon-flies. On such occasions they 

 retire to the shelter of the leeward side of a tree on the coming of a 

 puff so light that it causes only a gentle movement of the leaves. 



On the other hand, if conditions are suitable for high speed 

 flight the presence of wind appears to be helpful rather than 

 harmful. The speed of flight appears to increase with the strength 

 of the wind. In such "soarable" winds, dragon-flies travelling up 

 wind may be seen to glide ahead with a distinct increase of speed 

 whenever they are struck by a gust. A similar increase of speed on 

 entering a gust occurs in the case of cheels and other soaring birds. 



In low speed flight the wings of dragon-flies are in the "up" 

 position, the wing tips being on a higher level than the body. . 

 The tips of the front pair of wings are more elevated than those of 

 the hind wings. Hence the gliding dragon-fly has a distant re- 

 semblance to a staggered biplane. The abdomen is elevated above 

 the level of the thorax (Fig. 2). 



In high speed flight the abdomen appears to be generally if 

 not always horizontal and the wings appear to be less elevated 

 than when flight is at low speed. 



IX. Conclusions. 



Various species of dragon-flies have been found to possess the 

 power of soaring flight. 



Facts have been adduced that are incompatible with the idea 

 that this soaring flight is due either to undiscovered wing move- 

 ments or to the use of ascending currents. 



Soaring flight is now known to occur in three different classes 

 of animals, namely dragon-flies, flying-fishes and birds. It is re- 

 markable that, despite their widely different structure, size, and 

 weight, and the very different conditions under which they soar, 

 there should be such similarity of the flight in these three classes 

 of animals. In each class evidence is available that low speed 

 flight depends on the presence of sunshine and high speed flight 

 on the presence of wind. The speeds attained in the three classes 

 are comparable if not identical. 



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