462 Dr Hankin, The soaring flight of dragon-flies 



III. Object of flight with abdomen hanging down. 



A clue to the meaning of the adjustment was obtained by 

 observing instances in which it occurred in single individuals while 

 others near by were not using it. 



On several occasions it has been noticed that dragon-flies with 

 abdomens down show less speed than others whose abdomens 

 are up. With various species of dragon-fly gUding over water it 

 has been repeatedly noticed that the abdomen is lowered when 

 they need to check speed, either to avoid another dragon-fly 

 or on turning as they reach some obstruction. In a light wind 

 dragon-flies gliding to and fro over a restricted course, as is usual 

 with Pantala, have been seen to have abdomens down when going 

 with the wind and to raise their abdomens each time they go 

 in the opposite direction. 



A dragon-fly has been observed to lower the abdomen for a 

 moment, apparently to check speed, while catching an insect. 



We are thus led to the conclusion that lowering the abdomen 

 and hind legs is an adjustment intended to check speed. It is 

 obvious that in the lowered position the abdomen and legs must 

 present more resistance to speed ahead than occurs when these 

 organs are in the up position. 



IV. Conditions under which continued flight occurs 

 with abdomen down. 



From the diary of my observations it soon appeared that this 

 adjustment did not occur on days when the presence of thin 

 cirrus cloud had been noted. 



On one occasion it was noticed that the dragon-flies kept 

 changing the position of their abdomens from up to down every 

 few minutes. Suspecting the cause of this I obtained several 

 pieces of coloured glass through which the sun could be observed 

 without inconvenience. It was found that smaE thin clouds were 

 passing over it which were too small to cast a shadow or even to 

 cause any appreciable decrease in the intensity of the sunshine. 

 Whenever one of these clouds was over the sun the dragon-flies 

 held their abdomens up, downwards whenever the sun was clear. 



It was further found that if small cumulus clouds are rapidly 

 crossing the sky, and if there is not much wind at ground level, 

 dragon-fiies ghde with abdomens up when there is shadow and with 

 abdomens down when there is sunshine. After the sun comes out 

 there is a short interval before the abdomens are lowered. If the 

 edges of the cloud shadow are sharp cut this interval may be 

 measured and this was done on three occasions the times being 

 22, 23 and 27 seconds. 



