Insects and their world 



Fig. 52 . The common bluebottle, Order Diptera, shown here in a drawing 

 by Terzi, has a neat array of thoracic bristles 



As on land, larval insects in the water make use of the abdomen, 

 \^Tiggling to propel themselves along. This is seen most strikingly in 

 the lan'a of the fly, Chaoborus, often called Corethra, which seems to 

 have two silver}' spots in its body. These are air-sacs connected wixh 

 the tracheal system, and by var\'ing the volimie of these the buoyancy 

 of the body can be controlled. It is a mistake to say, as some textbooks 

 do, that by this means the larva can maintain itself at any desired level. 

 Water is practically incompressible, and so its densit>' does not vary 

 much at moderate depths. The density- of the water does, however, vary 

 with temperature, and with mineral content. What the air-sacs do is to 

 adjust the buoyancy until the Chaoborus larva is only just a httle denser 

 than the water roimd about. If the lar\'a remains still it wiU sink slowly, 

 but by t%^'itching the body it can quickly rise to any desired level, and 

 ft*om there float slowlv down asain. 



Flight 



Three hWng groups of animals are able to fly: birds, bats and insects. 

 Fhing-fishes have wing-like flns, but use them mainly as gliding- 

 planes. Even if they mav stretch the gUde by flapping the fins once or 



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