How they perform the primary functions 



which dissolves away in the water, and is not replaced, so that the bubble 

 gradually gets smaller. Eventually the insect has to come to the surface 

 for another bubble (Fig. 21). 



An aquatic insect is obviously at a disadvantage if it has to keep 

 coming to the surface for air. The adult insects that do this are generally 

 aggressive creatures, and can defend themselves. Soft-skinned nymphs 

 and larvae are more vulnerable to attack by surface enemies, and they 

 are generally equipped to stay below for long periods, or even continu- 

 ously. To do this they have to be able to extract dissolved oxygen from 

 the water, and to get enough of this for an active life they have to make 

 use of special organs of diffusion called tracheal gills. These are thin- 

 skinned projections from the surface of the body, either sticking out- 

 wards, or sometimes inwards into the cavity of the rectum or posterior 

 part of the intestinal tube (Figs. 23, 24). They are well suppHed with 

 tracheae, as a network of fine branches which run together, and lead 

 into one of the big tracheal trunks. The tracheal system of such an 

 insea is entirely closed, but it still functions in the usual way by 

 carrying oxygen from the tracheal gill to the various parts of the body. 



Blood-gills are similar organs, which have no tracheae, and which 

 are merely filled with blood. These are believed to control the balance 

 of water and salts in the body, and not to be actively concerned with 

 respiration. 



It is seldom that an aquatic insect rehes exclusively on one method 

 of breathing. Mosquito larvae have a posterior siphon by which they 

 reach to the surface for air, but some mosquitoes can live entirely 

 submerged, so long as they do not move about actively. This shows that 

 some oxygen must be dissolved and circulated by the blood, though 

 Lewis (1949) has demonstrated the use of tracheal gills as shown in 

 Fig. 22. It seems that the blood of insects does not normally contain any 

 special oxygen-carrying substance, but some larvae of flies of the family 

 Chironomidae ('bloodworms') do have haemoglobin in the blood. Here 

 it seems to act as an emergency mechanism, enabhng the larva to 

 breathe in stagnant water, where the oxygen content is dangerously low. 



Feeding and Excretion 

 Insects are so diverse that some group or other has learned to live on 

 every conceivable kind of food. Among the Apterygota and Hemimeta- 

 bola, where the young forms are very much Hke the adults, the various 

 stages generally have a similar diet, and may five together in one colony : 

 e.g. many of the plant-bugs and plant-lice. The Holometabola, on the 



41 



