XVIIL THE HONEY BEE. 263 



their freight of oxygen are carried by a great vein to the 

 heart and are thence pumped to all parts of the body, so 

 that every cell and fibre may have oxygen brought to it. 

 Thus by the blood-circulation the oxygen is distributed. 

 But in insects it is different. The blood-circulation (the 

 blood of the bee is quite colourless and has no such blood- 

 discs as are to be found in us) takes little or no part in 

 the distribution of oxygen. The tracheal tubes into which 

 the spiracles open, themselves ramify through all parts of 

 the " squash " and carry the oxygen directly to the tissues. 

 It is therefore of course extremely important that these 

 tubes should be kept open and prevented from collapsing ; 

 hence they are lined with a chitinous tube the walls of 

 which are spirally thickened. Between the spiral thicken- 

 ings the tube is very thin and delicate, and easily tears ; 

 so that if a tube is ruptured a little spiral thread projects 

 from the broken ends. It used to be thought that the 

 twirls of the spiral, the elements of its corkscrewity, were 

 naturally separate, like the iron wire which is placed in 

 indiarubber gas-tubing. But this has been shown to be a 

 mistake. It is a continuous tube with spiral thickenings. 



As in all insects with well-developed powers of flight, 

 the air-tubes of the bee are in certain parts enlarged into 

 capacious air-sacs, a conspicuous pair of which occupy 

 much of the abdomen of the worker. These not only 

 form respiratory reservoirs, but enable the insect to alter 

 its specific gravity, as does the fish by means of its swim- 

 bladder. If you watch a tired bee when it alights after 

 prolonged flight you will see it panting. It is the abdo- 

 men, in which are the large air-sacs, that pulsates with 

 the rapid inspirations, the air entering and passing out 

 through the spiracles. 



Now since in us the sense of smell is localized in the 

 nose at the entrance of the breathing system, it was not 



