II FLIES WITH AQUATIC LARVAE 101 



is nothing to keep the float at the surface, and it 

 settles steadily to the bottom. 1 



It is by virtue of this contractile force of the 

 surface-film that the Gnat-larva is enabled to main- 

 tain itself, against gravity, at the surface of the water. 

 The tip of the respiratory siphon is provided with 

 five flaps, which can be opened or closed by attached 

 muscles. When open they form a minute basin, 

 which, though its walls are cleft, does not allow the 

 surface-film of water to enter. \Yhen closed, the air 

 within the siphon is unable to escape. At the time 

 when the larva rises to the surface, the pointed tips of 

 the flaps meet the surface-film and adhere to it. The 

 attached muscles then separate the flaps, and in a 

 moment the basin is expanded and filled with air. 

 The surface-film is now pulling at the edges of the 

 basin, and the pull is more than sufficient to counter- 

 balance the greater density of the body of the larva, 

 which accordingly hangs from the surface without 

 effort. When the larva is alarmed and wishes to 

 descend, the valves close, their tips are brought to 

 a point, and the resisting pull of the surface-film is 

 reduced to an unimportant amount. 



Swammerdam found it necessary, in explaining the 

 flotation of the larva of the Gnat, to suppose that the 

 extremity of its siphon was supplied with an oily 

 secretion which repelled the water. No oil-gland can 

 be discovered here or elsewhere in the body of the 

 larva, and indeed no oil-gland is necessary. The 

 peculiar properties of the surface-film explain all the 



1 I owe this pretty little experiment to the ingenuity of Dr. 

 Stroud, 



