76 INSECT PHYSIOLOGY 



is again restored to the air, it is greatly in arrears for oxygen; and in 

 order to pay off this 'debt', and oxidize the lactic acid and other sub- 

 stances that have accumulated, its rate of oxygen consumption re- 

 mains above the normal level for a long time (Fig. 11, B). During 

 the anaerobic period, the blood naturally becomes more acid, the 

 pH falling, in the case of the grasshopper, from 6-8 to 5-8, and the 

 carbon dioxide capacity is reduced; consequently, at very low ten- 

 sions of oxygen, there may be a liberation of combined carbon 

 dioxide. This type of temporary anaerobic metabolism is a normal 

 occurrence in the life of Gasterophilus (parasitic in the stomach of the 

 horse) which can survive as long as 17 days in the complete absence 

 of oxygen ; experimentally, it occurs in insects of all kinds. 



The anaerobic production of energy by this process of 'glycolysis' 

 can be utilized only in certain of the less active muscles of insects. It 

 will not serve to maintain flight. The flight muscles are very richly 

 supplied with tracheoles; their metabolism is strictly aerobic. They 

 build up almost no oxygen debt, and can function only so long as the 

 lactate and particularly the a-glycerophosphate produced by gly- 

 colysis can be completely and directly oxidized. 



Metabolism and chemical changes during growth and metamorphosis 



The curve of oxygen consumption during the pupal stage of insects 

 usually follows a more or less U-shaped course; it falls first and then 

 rises again before emergence (Fig. 11, A). The level of oxygen con- 

 sumption at any moment represents the sum of the oxygen required 

 by all the different tissues. What may be the chief organs or the chief 

 activities consuming oxygen at the different points along this curve 

 have not been fully worked out. But since the quantity of respiratory 

 enzymes present will run more or less parallel with the respiratory 

 requirements it is not surprising that the cytochrome system and the 

 substrate dehydrogenase systems follow a similar curve. An increas- 

 ing rate of metabolism may be evident in the developing pupa at a 

 time when almost no mechanical work is being done. The oxygen 

 concerned is required for endothermic synthesis, and perhaps mainly 

 for protein synthesis, which is an integral part of the growth process 

 and which demands a large amount of energy. 

 The changes in the total composition of the insect body during 



