282 MOTIONS OF INSECTS. 



If disposed to descend, it seems to sink by the weight of 

 its body ; but when it would move upwards again, it ef- 

 fects its purpose by alternate contortions of the upper 

 and lower halves of it, and thus it moves with much ce- 

 lerity. The laminae gr swimmers, which terminate its 

 anus a , are doubtless of use to it in promoting this pur- 

 pose. It does not, that I ever observed, move in a la- 

 teral direction, but only from the surface downwards, 

 and vice versa. Another dipterous larva (Coretkra culi- 

 ciformis), which much resembles that of the gnat in 

 form, differs from it in its motions and station of re- 

 pose. For, instead of being suspended at the surface 

 with its head downwards, it usually, like fishes, remains 

 in a horizontal position in the middle of the water. 

 When it ascends to the surface, it is always by means of 

 a few strokes of its tail, so that its motion is not equable, 

 sed per saltus. It descends again gradually by its own 

 weight, and regains its equilibrium by a single stroke of 

 the tail 5 . A well known fly (Stratyomis Chamaleori), in 

 its first state an aquatic animal, often remains sus- 

 pended, by its radiated anus, at the surface of the water, 

 with its head downwards. But when it is disposed to 

 seek the bottom or to descend, by bending the radii of 

 its tail so as to form a concavity, it includes in them a 

 bubble of air, in brilliancy resembling silver or pearl ; 

 and then sinks with it by its own weight. When it 

 would return to the surface it is by means of this bubble, 

 which is, as it were, its air-balloon. If it moves upon 

 the surface or horizontally, it bends its body alternately 

 to the right and left, contracting itself into the form of 

 the letter S; and then extending itself again into a 



a Reaum. iv. t. 43, /. 3. nn. b De Geer, vi. 375. t. xxiii./ 4, 5. 



