238 



MOTIONS OF INSECTS. 



furnished with swimmers or anal appendages, by means of 

 which they are enabled to swim ; the other have them not, 

 and hence are not able to rise from the bottom. 1 The larvae 

 of Dytisci, by means of these natatory organs, will swim, 

 though slowly, and every now and then rise to the surface 

 for the sake of respiration. Those of Ephemera, when they 

 swim, apply their legs to the body, and swim with the swift- 

 ness and motions of fish. 2 Those of the true May-fly (Sialis 

 lutaria), on the contrary, use their legs in swimming, and at 

 the same time, by alternate inflexions, give to their bodies 

 the undulations of serpents. 3 But the larvae of certain 

 dragon-flies (Aeshna and Libellula) will afford you the most 

 amusement by their motions. These larvae commonly swim 

 very little, being generally found walking at the bottom on 

 aquatic plants : when necessary, however, they can swim 

 well, though in a singular manner. If you see one swimming, 

 you will find that the body is pushed forward by strokes, 

 between which an interval takes place. The legs are not 

 employed in producing this progressive motion, for they are 

 then applied close to the sides of the trunk, in a state of 

 perfect inaction. But it is effected by a strong ejaculation 

 of water from the anus. When I treat upon the respiration 

 of insects, I shall explain to you the apparatus by which 

 these animals separate the air from the water for that pur- 

 pose; in the present case it is subsidiary to their motions, 

 since it is by drawing in and then expelling the water that 

 they are enabled to swim. To see this, you have only to 

 put one of these larvae into a plate with a little water. You 

 will find that, while the animal moves forward, a current of 

 water is produced by this pumping in a contrary direction. 

 As the larva, between every stroke of its internal piston, has 

 to draw in a fresh supply of water, an interval must of 

 course take place between the strokes. Sometimes it will 

 lift its anus out of the water, when a long thread of water, if 

 I may so speak, issues from it. 4 



1 Miger, Ann. du Mus. xiv. 441. 2 De Geer, ii. 621. 



3 Ibid. 725. 



4 De Geer, ii. 675. Compare Reaum. vi. 393. 



