314 A BOOK OF INSECTS 



extended to a distance of several inches. By this means 

 the submerged larva secures a supply of oxygen, for 

 the tube contains two tracheae, which lead to spiracles ; 

 while if the liquid in which the larva lies becomes deeper, 

 owing to a sudden shower of rain, the tube can be 

 lengthened accordingly. 



All the insects which have so far been mentioned 

 drown quickly if they are forced to remain under water 

 for more than a limited period of time. They are equipped, 

 as it were, with a more or less perfect diving apparatus ; 

 but they are entirely dependent upon atmospheric air. 

 There are other insects, however, which can remain below 

 indefinitely, for they are able to utilise "dissolved air" — 

 i.e. the air which is mixed with water. In the very young 

 nymphs of many species, such as may-flies and some 

 dragon -flies, aeration of the blood is effected through the 

 skin. But as growth proceeds, specialised gills are 

 frequently developed. These are very diverse in form 

 and situation, but in general terms they may be described 

 as thin-walled outgrowths of the integuments containing 

 delicate branches of the tracheal system. More rarely 

 the gills contain only blood. By a process which is 

 at present imperfectly understood, these organs extract 

 oxygen from the water. The larvae of whirligig beetles, 

 of alder-flies, and the nymphs of may-flies have branching 

 or leaf-like gills at the sides of the abdominal segments. 

 Other species carry their gills like tails at the end of 

 the body. But in certain of the large dragon-fly nymphs 

 the gills occupy the posterior part of the alimentary 

 canal, into which water is drawn and expelled by a gentle 

 pulsation of the abdomen. When alarmed, these nymphs 

 can eject the water with such force that their bodies are 

 propelled swiftly forward. Other dragon-fly nymphs 

 possess exposed tracheal gills at the tail-end of the body. 



