205 



Headlee (T. J.). Some facts relative to the Influence of Atmospheric 

 Humidity on Insect Metabolism. — Jl. Econ. Entom., Concord, N.H., 

 X, no. 1, February 1917, pp. 31-38. 



The effect of atmospheric moisture upon the rate of insect meta- 

 bolism has been proved to be extremely variable, in some cases re- 

 tarding development and in others hastening it. In experiments 

 undertaken with the bean weevil, Bruchus obtectus. Say, a study 

 was made of the variations in the rate of metabolism in response to 

 atmospheric humidity in the different stages of a single species entirely 

 dependent upon metabolic water, and of the miderlying cause of the 

 response. In order that such factors as temperature and light or the 

 presence of carbon dioxide and oxygen might not influence the results, 

 an effort was made to eliminate them as variables, all insects being 

 kept at a temperature of 80° F., in complete darkness, subjected to 

 a passing stream of air, and the experiments were carried on 

 simultaneously with a view to eliminating as far as possible the 

 variable barometric pressure. The experiments are described in 

 detail and tables of the results given. The conclusions reached were 

 that the rate of metabolism in the pupae of both the bean weevil 

 and the Angoumois grain moth, Sitotroga cerealelki, Oliv., varies 

 inversely \\'ith atmospheric humidity ; in the adult of the former 

 it varies with, while in the adult of the latter it varies inversely as 

 the humidity ; in the egg-stage the speed of metabolism varies inversely 

 with the humidity ; in the larvae it varies with the humidity ; in 

 the life-cycle as a whole the rate varies with the humidity. 



Among insects dependent upon metabolic water alone, this response 

 to atmospheric moisture leads to the supposition that for each stage 

 of the insect there is a definite internal water optimum, that is, an 

 amount of body fluid which will permit necessary chemical and 

 physical changes to take place mth the greatest ease and speed. 

 Atmospheric humidity apparently acts upon the insect by the removal 

 of water and also by the prevention of the loss of body fluid. The 

 relation of the supply of body fluid to the optimum in any specific 

 stage of an insect is therefore the underlying cause of the effect 

 of atmospheric moisture. Continued exposure to dry air ^\^ll reduce 

 the body fluid imtil the tissues become so poorly supplied that living 

 processes can no longer continue, and death follows. This is well 

 shown in the destruction of the bean weevil larvae and the pupal 

 stages of the Hessian fly [Mayetiola destnwtor] by drought. 



The inability of the bean weevil to reproduce in moisture of 26 per 

 cent, or below led to an examination of dry air as a means of sterilising 

 and preserving bean seeds from weevil injury. Experiments showed 

 that in a sealed chamber about 5 cc. of concentrated sulphuric acid 

 per 1,000 cc. of air prevents reproduction when a limited number of 

 beans is used. This amount has to be increased in proportion to the 

 moisture which the beans can give off. Germination tests showed 

 that a prolonged exposure of 92 days resulted in injury to the beans. 

 The period required for sterilisation does not however exceed 30 days 

 and this would not affect the vitality at all. The native Rhodesian 

 method of protecting maize from weevils by mixing the grain with 

 finely powered wood ashes is probably a practical application of the 

 effect of low relative humidity. 



