SURROUNDINGS OF GROWING INSECTS 195 



by means of it, they can get into direct touch with the atmos- 

 phere and do not require special gills for breathing dissolved 

 air. This mode of respiration has already been mentioned in 

 connexion with the larva of the carnivorous water-beetles 

 (Dyticus, see pp. 101-2). We may now turn to certain larvae 

 of Diptera that are able thus to draw on the atmospheric 

 source for their needed oxygen. 



In an earlier chapter (pp. 130, 133) we saw that many 

 dipteran larvae have the functional spiracles restricted to a pair 

 at the hinder end of the body ; such an arrangement is advan- 

 tageous for a maggot living buried in some decaying organic 

 substance. This position of the spiracles is clearly a good 

 adaptation for enabling larvae living submerged in water to get 

 into contact with the air by thrusting the tail-region up to the 

 surface. As an example of such an adaptation we may take 

 the familiar larva of a common gnat (Culex). Gnats or 

 mosquitoes as they are often called are notorious insects on 

 account of the unpleasant habit of blood-sucking practised 

 by their females which are provided with formidable lancet - 

 like mandibles and maxillae, capable of piercing the skin and 

 giving the insect's suctorial labium access to blood. The 

 female gnat (Fig. 102 a) lays her eggs on the surface of stag- 

 nant water, the eggs being associated in a small mass known 

 as the " egg-raft " because it floats each egg (b), with its long 

 axis vertical, opening by a little circular lid at the lower end 

 so that the newly-hatched larva dives at once into the water. 

 The larva (Fig. 102 c) has a relatively large head with short 

 feelers, prominent fringed " combs " or " brushes ", like those 

 of the Simulium larva described above, which sweep particles 

 of food into the mouth, and the typical insectan mandibles and 

 maxillae. The thorax is very broad, its three constituent 

 segments not clearly distinguishable. The abdomen is narrow 

 and sub-cylindrical, its terminal segment, through which the 

 intestine opens, bearing long bristles and stiff plate-like pro- 

 cesses. From the last segment but one diverges a dorsal cylin- 

 drical outgrowth (sp) through which the tracheal trunks run,, 

 a spiracular siphon with the apertures of the air-tubes at its 

 tip, guarded by a group of little, pointed processes which can 

 be brought together at their slender apices or spread out 

 somewhat like petals of a flower. The gnat-larva swims 



