292 MOTIONS OF INSECTS. 



one 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^. 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 respira- 

 tion. Those of Ephemera:, when they swim, apply their 

 legs to the body, and swim with the swiftness and mo- 

 tions of fish''. 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 ^ But the larvae of certain 

 dragon-flies (^Aes/i?ia and Libcllula,) will afford you the 

 most amusement by their motions. These larvae com- 

 monly 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 mo- 

 tions, 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 



•^ Miger, Arm, du Mux, xiv. 441. '' De Gecr, ii. 621. 



' Ibid. 725— 



