Insects and their world 



and fro. This renews the air more completely. Furthermore, the spiracles 

 can be closed down temporarily to reduce the loss of water- vapour from 

 the body. The tough integument of an insect does not lose much water, 

 but the air expelled from the spiracles carries out water-vapour, just 

 as our own breath does on a winter's morning. In very dry cUmates this 

 loss of water might be fatal to the insect if it could not be regulated. 



The carbon-dioxide produced by the insect escapes readily through 

 the integument, and is not controlled by the respiratory movements. 



The system just described is excellent for insects with access to un- 

 limited air, but obviously will not work for insects Hving in water, or 

 in very wet places, or as internal parasites in other animals. These must 

 get their oxygen either by reaching to the open air with their spiracles, 

 or by absorbing dissolved oxygen from the fluids surrounding them. 

 Thus larvae of blowflies (maggots) may feed in suppurating wounds, 

 while keeping the posterior spiracles in contact with the air. The 

 parasitic larvae of botflies and warbleflies, living in the intestine and 

 imderneath the skin, respectively, use both methods. They take up 

 oxygen through the skin, and also make contact with air-spaces among 

 the tissues or among the food of the host animal. 



Fig. 22. The aquatic larva of a culicine 

 mosquito lying upside down on the bottom. 

 The arrows indicate a current produced by 

 the mouth-brushes, which carries minute 

 organisms as food to the mouth, and also 

 ensures a flow of oxygenated water over 

 the tracheal gills at the tip of the abdomen. 

 After Lewis, 1949 



Many larvae that live in water, or in decaying, semi-liquid organic 

 matter, have developed a siphon or breathing-tube as an extension from 

 the posterior spiracles, and use it to reach out to the air (Fig. 45). 

 Larvae of Donaciine beetles and of some mosquitoes and other flies, 

 tap the air that occurs between the cells in the stems of water-plants, by 

 piercing the plant with a pointed siphon. 



38 



