28o THE BIOLOGY OF INSECTS 



this point is admirably elucidated in Miall's well-known 

 book (1895). He points out how in the common gnats 

 (Culicidae) the surface tension has a part to play in every 

 stage of the life-history. The eggs are laid on the water — « 

 over two hundred glued together to form the familiar 

 *' raft." Each egg is elongate, broad at one end with a 

 circular outline, narrow at the other ; the nsrrow pointed 

 ends are so close together that the water cannot get between 

 them ; therefore, as they are always separated by air-spaces, 

 they must remain upwards as the raft floats, and the sub- 

 merged end of each egg opens by a round lid to allow the 

 newly hatched larva to dive into the water. The larva 

 has in connection with its hindmost spiracles, which alone 

 are functional, a prominent tube diverging from the tail-end 

 of the body ; the opening of this tube is surrounded by a 

 set of pointed flaps which close it when brought together. 

 The larva, rising tail-first to the surface, pierces the film 

 with the adjacent points of these flaps, and then separates 

 them so that they form at the surface a cup-like depression 

 leading into the spiracle tube. Areas of film are held 

 between adjacent edges of the flaps, so that the gnat grub 

 hangs as it were from the surface supported by the tension 

 of the film which is pulled downwards by the drag of the 

 larva, but not broken. When the spiracular valve is closed 

 by the drawing together of its pointed flaps, the tips of the 

 processes can be disengaged from the surface and the larva 

 dives, its air-tube system completely shut off from the 

 surrounding water. (See Fig. 87, b.) 



The restriction of the gnat grub's functional spiracles 

 to the hinder end of its body enables it — Hke many other 

 larvae of Diptera — to breathe at its tail while it feeds with 

 its mouth. The gnat pupa, on the other hand, has a pair 

 of respiratory trumpets on the prothorax projecting dorsal- 

 wards. By means of these it can hang and breathe from 

 the surface-film, the strongly arched dorsal region of its 

 thorax uppermost and its abdomen pointing downwards. 

 This attitude is in preparation for the emergence of the 

 gnat into the atmosphere, for the pupal cuticle slits length- 



