MOTIONS OF INSECTS. 285 



lis history. Its larva is a very singular creature, fur- 

 nished with a remarkable anal apparatus for respira- 

 tion, by which it usually remains suspended at the sur- 

 face of the water. If disposed to descend, it seems to 

 sink by the weight of its body ; but when it would 

 move upwards again, it eflects its purpose by alter- 

 nate contortions of the upper and lower halves of it, 

 and thus it moves with much celerity. The laminae or 

 swimmers, which terminate its anus'', are doubtless of 

 use to it in promotitig this purpose. It does not, that 

 I ever observed, move in a lateral direction, but only 

 from the surface downwards, and vice versa. — Another 

 dipterous larva (Corethra cultciformis, Meig.) which 

 much resembles that of the gnat in form, differs from 

 it in its motions and station of repose. For, instead of 

 being suspended at the surface with its head down- 

 wards, 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, scd per 

 saltus. It descends again gradually by its own weight, 

 and regains its equilibrium by a single stroke of the 

 taiP. — A well known fly (Stratj/omis Chama'leon, F.), 

 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 dis- 

 posed 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 



* Reaum. iv. t. 43. /. 3. nn. *" De Ciccr, vi. 375. I. xxiii. f. 4,5. 



