MOTIONS OF INSECTS. 351 



filled by a pure, transparent, glassy membrane. In two 

 of the genera belonging to this tribe, the wings, when 

 the animal is at rest, are always expanded, so that they 

 can take flight in an instant, no previous unfolding of 

 these organs being necessary. In Agrion, the other 

 genus of the tribe, the wings when they repose are not 

 expanded. I have observed of these insects, and also 

 of several others in different orders, that without turning 

 they can fly in all directions backwards, and to the 

 right and left, as well as forwards. This ability to fly 

 all ways, without having to turn, must be very useful to 

 them when pursued by a bird. Leeuwenhoek once saw 

 a swallow chasing an insect of this tribe, which he calls 

 a Mordella, in a menagerie about a hundred feet long. 

 The little creature flew with such astonishing velocity- 

 to the right to the left and in all directions that this 

 bird of rapid wing and ready evolution was unable to 

 overtake and entrap it ; the insect eluding every attempt, 

 and being generally six feet before it a . Indeed, such is 

 the power of the long wings by which the dragon-flies 

 are distinguished, particularly in JEshna and Libellula, 

 and such the force of the muscles that move them, that 

 they seem never to be wearied with flying. I have ob- 

 served one of the former genus (Anax Imperator, Leach) 

 sailing for hours over a piece of water sometimes to and 

 fro, and sometimes wheeling from side to side ; and all 

 the while chasing, capturing, and devouring the various 

 insects that came athwart its course, or driving away its 

 competitors without ever seeming tired, or inclined to 

 alight. Another species (JEshna variegata), very com- 

 mon in lanes and along hedges, which flies, like the 

 a Leeuw. Epist, 6. Mart. 1717- 



