374 THE TRACHF.AL SYSTEM Ol- THE IMAGO. 



mammals. In man the amount of oxygen absorbed is less 

 than .juVifth of the body weight per hour. 



It must be observed, however, that in warm-blooded animals 

 the proportionate amount of oxygen to the body weight in- 

 creases very rapidly as the size of the animal experimented 

 on diminishes ; thus in the Dormouse it equals ;; Vth of the 

 body weight per hour, and in small birds it is even as much 

 as -Jyth, therefore it is clear that the comparison of the oxygen 

 absorbed to the bod)- weight of those warm-blooded vertebrates 

 which approach most nearly to insects in size leads to a very 

 different conclusion. 



On the other hand, insects absorb about twenty times as 

 much oxygen as cold-blooded vertebrates Frogs and Lizards 

 weight for weight, from 1,000 to 2,000 times more than the 

 higher Mollusca, and 200 to 400 times as much as the higher 

 Crustacea, if the observations recorded by various observers 

 are correct. 



The respiratory quotient, or the relation of the CO> given 



off to the O absorbed ( ' = '79) is low; in man it is given 



as o'g. This, perhaps, indicates that the proteid metabolism 

 is more rapid, but it is capable of other interpretations, as, 

 for example, that more hydrogen is oxidized in proportion to 

 carbon ; but our knowledge is too limited to justify any con- 

 clusions on this point. 



As a single Cockchafer (Mclolontha vulgar is) absorbs all the 

 oxygen contained in seven cubic centimetres of air in an hour, 

 the least quantity of air which must pass into the trachea; in 

 this time is 7 cc. = 7,000 cubic millimetres, and that on the 

 supposition that the \vhole of the oxygen contained in the in- 

 spired air is absorbed. 



The spiracular sacs of the thorax are six in number, and I 

 estimate their united cubic contents to be equal to about 6 

 cubic millimetres. The spiracular sacs of the abdomen are 

 twelve in number, and I do not think their united capacity ex- 

 ceeds 2 cubic millimetres. The number of respiratory contrac- 

 tions of the spiracular sacs may be estimated at about 120 per 



