February, '17] HEADLEE: HUMIDITY AND METABOLISM 35 



that were illustrated at the beginning of this paper and lend support 

 to the theory which was partly set forth by Bachmetjew.^ "Appar- 

 ently there is a degree of atmospheric humidity, which being the most 

 favorable to the maximum speed of insect metabolism should be 

 designated as the optimum; that this optimum varies for each species, 

 for each stage of each species, and for each stage of each individual. " 



When considering this question it must be remembered that we are 

 dealing, in this instance, with species which are totally unable to 

 imbibe and which must depend entirely upon metabolic water. Ac- 

 cording to Babcock^ the sources of metabolic water are mainly three, — 

 oxidations of food and tissues, breaking up molecular structure into 

 new forms of a lower order, and changing the molecular structure of 

 substances composing its nutrients and tissues without reducing their 

 complexit}^ The first two produce CO2 and all three produce meta- 

 bolic water. It must also be remembered that when dealing with an 

 insect or with insect stages which can imbibe the maintenance of the 

 water optimum against the influence of evaporation is easy and that 

 variations in atmospheric moisture may under such conditions prove 

 ineffective as shown in the case of Toxoptera graminum. 



Among insects or insect stages dependent upon metabolic water 

 alone, this response to atmospheric moisture can best be accounted 

 for on the supposition that for each stage of each insect there is a 

 definite internal water optimum — an amount of body fluid which will 

 permit necessary chemical and physical changes to take place with the 

 greatest ease and speed. It would seem that atmospheric humidity 

 can act upon the insect directly in two ways — by the removal of water, 

 and by the prevention of the loss of body fluid. 



Under the terms of this theory the direct effect of atmospheric 

 moisture upon the specific stage of a specific insect depends upon the 

 relation of the supply of body fluid to the optimum. If it be well 

 above, dry air will remove the surplus and cause the metabolism to 

 speed up as in the pupa or egg stage of the bean weevil, while moist 

 air by preventing the rapid reduction of body fluid will prolong the 

 stage. If the body fluid be just about the optimum, dry air will 

 reduce it below that point and decrease the speed of metabolism while 

 moist air will prevent the decrease of body fluid and shorten the time 

 required to complete the stage of development as illustrated by the 

 larvse of the bean weevil. 



Continued exposure to dry air will reduce the body fluid until the 

 tissues become so poorly supplied that living processes can no longer 



1 Bachmetjew, P., 1. c, p. 689. 



^ Babcock, S. M., Wisconsin Research Bull. No. 22, Wis. Agric. Expt. Sta., pp. 

 88-89. 



