THE EESPIRATORY SYSTEM. 115 



function as intermediaries between the tracheae and the cells, but 

 Kosclievnikov (1900) has shown that these cells appear to be tem- 

 })orary storehouses for waste products from the tissues — presunuibly 

 uric acid compounds which have been already oxidized. Even the 

 fat-body has been regarded as a sort of lung in which oxidation takes 

 place, but there is no evidence to support this theory, although, for 

 that matter, there is little evidence in favor of any theory in insect 

 physiology. 



The process of metabolism, or the vital activity of the cells them- 

 selves, results in a breaking down of the complex and highly unstable 

 protoplasmic molecules into chemical substances of much simpler con- 

 struction, and it is these by-products of metabolism that are attacked 

 by the oxygen in the blood furnished by the respiratory system. Pro- 

 toplasm consists principally of the elements carbon, oxygen, hydro- 

 gen, and nitrogen, and the oxidation process results, as before stated, 

 in the formation of carbon dioxid (CO^,) and water (H^O), while 

 the residuary products are mostly organic compounds of nitrogen 

 related to uric acid (C^H.N.O;,,) and urea (CON.H,). The carbon 

 dioxid is a soluble gas which diffuses into the end tubes of the tracheae 

 and is exhaled. A part of the water at least is given olf with the 

 " breath " in the form of water vapor, for drops of it can be collected 

 by inclosing bees or any insects in a tube for a short time. The nitro- 

 gen compounds and probably a part of the water are dissolved in the 

 blood and removed by the Malpighian tubules, which are the kidneys 

 of insects. 



Besides this oxidation of waste products, which allows the process 

 of metabolism to go on unhindered, the inhaled oxj-gen serves also 

 another purpose, namely, that of maintaining the body heat. Al- 

 though insects are usually classed as " cold-blooded " animals, they 

 nevertheless maintain a temperature which is always higher than 

 that of the surrounding air and is often a number of degrees above 

 it. It is well known that the temperature of a beehive during the 

 brood-rearing season is almost as high as that of the human body, 

 and that even during winter it remains at nearly SO'' F. ; but this is, 

 of course, due to the accumulation and condensation of the warmth 

 from the bodies of a great many bees, and is much higher than the 

 temperature of any bee outside of the hive. In our own bodies 

 certain substances are consumed by oxidation in the blood simply 

 to i^roduce the necessary heat energy for maintaining metabolism, 

 and hence it seems reasonable to suppose that the same thing takes 

 place in insects, although of course to a much less degree. 



There are generally ten pairs of spiracles or breathing apertures 

 in insects, two being situated on the sides of the thorax between 

 the segments, but probably belonging to the mesothorax and the 



