114 THE ANATOMY OF THE HONEY BEE. 



cells lined with a delicate layer of chitin. The latter, however, is 

 strengthened by circular thickenings which give the appearance of 

 an internal spiral thread, but a closer examination shows that each 

 thickening makes only a few turns and that several lie in parallel 

 bands. This structure is for the purpose of maintaining an open 

 passageway for the air through the very thin-walled tubes. The 

 tracheae branch into fine capillaries and these terminate in excessively 

 delicate end-tubes. In some cases it is easy to see that a great 

 number of capillary branches surround the cells of a tissue, if 

 they do not actually enter the cell walls, but in others it can not be 

 shown that the tracheae really penetrate below the surface of a mass 

 of cells. 



Gases in solution, like solids, pass freely back and forth through 

 moist animal membranes, going in the direction of the least pressure 

 of each particular gas. By this simple method the gases go back and 

 forth through the walls of the gills, lungs, or air tubes and permeate 

 the tissues themselves. Vertebrate animals, as already explained, 

 have a red substance in the blood called hemoglobin which has a 

 very great oxygen-absorbing power and which greatly increases the 

 oxygen-carrying power of the blood, but still a certain amount of 

 oxygen is carried in solution by the liquid or plasma of the blood. 

 Now, tl\,e blood of insects has none of this hemoglobin and all the 

 oxygen it can carry is that which dissolves in its plasma, but, on 

 account of the extensive ramification of the air tubes, it is not neces- 

 sary for the blood to distribute the oxygen to the organs. It is usually 

 stated that the blood in insects does not carry oxygen at all, ^cept 

 for its own use, but it would seem physically impossible that the gases 

 should not diffuse out of the fine terminal air-tubes into the blood 

 when they do so in all other cases. If the blood of a crab or crayfish 

 is capable of carrying enough oxygen in solution to supply the wants 

 of the body, there is no reason why that of an insect, which has much 

 better facilities for obtaining air, should not do the same. Further- 

 more, we can not suppose that the products of katabolism have to 

 accumulate about the end tracheae in order to be oxidized. They are 

 produced wherever metabolism is going on, which is everywhere in 

 the living cell substance, and, hence, the latter must be permeated 

 with oxygen in solution, which must also be in the blood along with 

 the carbon dioxid formed. The carbon dioxid diffuses back into the 

 end tracheae from the blood. Therefore, while the great extent of the 

 tracheal system in insects relieves the blood of the work of distribut- 

 ing the oxygen, the blood must nevertheless serve as an intermediary 

 medium for both the oxygen and the carbon dioxid between the fine 

 terminal tracheal branches and the cells. 



It has sometimes been suggested that certain large cells called 

 cenocytes, found especially in connection with the tracheal system, 



