liEAVES FROM MY NOTE-BOOK. 139 



sweeping minute organisms into its mouth with its vibratile cilia, 

 whilst at the same time it breathes uninterruptedly through the 

 respiratory siphon attached to the eighth segment of its abdomen. 

 If startled, the larva sinks slowly to the bottom by gravity alone, 

 which shows that the body is denser than water. How then is it 

 possible for a larva heavier than water to remain floating at the 

 surface without effort ? The possibility of such a thing turns upon 

 the existence of the surface film, formed by the same contractile 

 force which rounds the rain-drop and the air-bubble. The tip of 

 the gnat larva's 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 to enter. At the time when the 

 larva puts itself in position to begin its feeding operations, the 

 pointed tips of the flaps meet the surface film and adhere to it. 

 The attached muscles 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 this pull is more than 

 sufficient to counteract 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. In its 

 pupal stage the gnat breathes through two respiratory trumpets 

 placed near the head, in such a position that, when the pupa is at 

 rest, the margins of the trumpets come flush with the level of the 

 water. The tail end is now modified as a swimming fan. 



The gnat at all stages requires plenty of air, and its egg-raft, 

 containing from two hundred and fifty to three hundred eggs, is 

 as ingeniously contrived for aeration, as are the contrivances at all 

 other stages of the insect's life. If we take two or three of these 

 tiny egg-rafts, and place them in a jug of water, we may pour the 

 water into a basin again and again; the rafts float instantly to the 

 surface, and the moment they come to the top they are seen to be 

 as dry as at first. The fact is that the surface film cannot penetrate 

 the fine spaces between the pointed ends of the eggs. The cavity 

 of the egg-raft is thus over-spread by an air-bubble when accident- 

 ally submerged. The eggs are kept from contact with water ; the 



