MOTIONS OP INSECTS. 443 



again swim about in cases, or crawl over the submerged bottom ; and 

 others walk even on the surface of the water. I shall not now enlarge on 

 all these kinds of water-motion, since many will come under consideration 

 hereafter. 



There are two descriptions of larvae of HydropMlidcB, 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 larvas of Dytkci, 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 Ej^hemercB, when they swim, apply theit 

 legs to the body, and swim with the swiftness and motions 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 (Aeshna and LibeUiilci) will aiford 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 respira- 

 tion of insects, 1 shall explain to you the apparatus by which these 

 animals separate the air from the water for that purpose; 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. * 



II. I am next to say something upon the motions of insects in their 

 pujici state. This is usually to our little favourites a state of perfect 

 repose ; but, as I long since observed, there are several that, even when 

 become pupae, are as active and feed as rapaciously as they do when they 

 are either larvae or perfect insects. The Dermaptern, Orthoptcra, Hevii- 

 ptera, many of the Neuroptera, and the majority of the Aptcra, are of this 

 description. With respect to their motions, we may therefore consider 

 pupae as of two kinds — active pupjE, and quiescent pupae. 



The motions of most insects whose pupae are active are so similar in all 

 their states, except where the wings are concerned, as not to need any 

 separate account. I shall therefore request you to wait for what I have to 

 say upon them, till I enter upon those of the imago. One insect, however, 



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



3 Ibid. 725. 



* Ibid. ii. 675. Compare Reaum. vi. 393. 



