460 Dr Hankin, The soaring flight of dragon-flies 



The soaring flight of dragon-flies. By E. H. Hankin, M.A., 

 Sc.D., Agra, India. 



{Read 16 May 1921.] 



I, Introduction. 

 Dragon-flies of the larger species to be found in Agra, such as 

 Hemianax ephippiger, frequently spend the day gliding at a height 

 of several metres above tree-top level. At sunset they come down 

 to low levels in search of prey, and hence have been described as 

 of crepuscular habits. When flying at a height during the day 

 they may be watched through a binocular till one's arms are tired 

 without a single flap of the wings being observed. Proofs that such 

 flight is not due to undiscovered wing movements will be found 

 in a later paragraph, thus leading to the conclusion that it is an 

 instance of soaring flight. Smaller species, such as Pantalaflavescens 

 and Tramea burmeisteri, that habitually fly nearer to ground level 

 are more suitable for detailed observation and on these most of 

 my work has been done. 



II. Soaring flight at low speed. 



The mode of flight of Pantala flavescens during the months of 

 July and August and in fine weather is as follows. 



If, in the early morning, one watches a group of these dragon- 

 flies, it will be seen that their flight consists of alternate periods 

 of flapping and gliding. The flaps at this time are of the hind 

 wings only. As the sun gets stronger the amount of flapping is 

 seen to decrease and to be more and more replaced by gliding. 

 By about nine o'clock the dragon-flies are showing two methods 

 of flight; either they glide with the abdomen horizontal, aided by 

 two or three flaps of the hind wings every two or three yards of 

 their course (Fig. 1), or, on the other hand, they glide with the 

 abdomen elevated, travelling horizontally for the most part, and 

 apparently may proceed in this way for indefinite distances 

 (Fig. 2). Glides of 10 or 15 seconds without a flap may be observed. 

 Flaps are apt to occur when two dragon-flies happen to approach 

 each other. In the case of another species (Rhyothemis variegata), 

 observed in Calcutta, proofs were obtained that such occasional 

 flapping of their brilliantly coloured hind wings is used as a signal 

 to other individuals [Animal Flight, p. 388). 



The mode of flight continues to vary with the time of day. If 

 there is strong sunshine and but little wind, then, at about eleven 

 o'clock, the dragon-flies may be seen gliding with the abdomen 

 and hind legs hanging down as shown in Fig. 3. This adjustment 

 is maintained till about three o'clock when the abdomen is seen 



