SHORTER ARTICLES AND DISCUSSION 



HUMIDITY — A NEGLECTED FACTOR IN ENVIRON- 

 MENTAL WORK 



An admittedly rough but probably fair estimate of the relative 

 interest which has been taken in the relation of the various 

 environmental factors to insects, at least, may be made from the 

 fact that Bachmetjew in his admirable compilation 1 of the work 

 along these lines devotes, in round numbers, four hundred pages 

 to temperature, one hudred and fifty to food and chemicals, 

 seventy to light, forty-five to humidity, fifteen to electricity and 

 magnetism and thirty to mechanical and other factors. Why is 

 it that temperature is given about a third more attention than all 

 the other factors put together? Is it true that it is nearly ten 

 times as interesting or important as humidity ? 



A partial answer to the first question undoubtedly is that tem- 

 perature is easily controlled as well as measured, whereas humid- 

 ity, for example, is not easily controlled and the means of 

 measuring humidity in small containers are untrustworthy and 

 expensive. Furthermore, work with temperature gives results. 

 The unfortunate part is that these results have usually been as- 

 cribed wholly to temperature. 



In the course of some work at the Carnegie Station for Ex- 

 perimental Evolution I found that I could change to a surprising 

 extent the markings on the larvae of a moth (Isia Isabella) by 

 varying the temperature at which they fed and moulted. How- 

 ever, such changes were much more definite when the tempera- 

 ture was kept constant and humidity varied. I did not have the 

 necessary apparatus for getting accurate control of either factor, 

 but I feel confident that temperature had little or no direct in- 

 fluence. It was acting through its influence upon humidity. 



It would seem unnecessary to urge upon experimenters such a 

 fundamental principle in the logic of cause and effect, but the 

 fact is that with only two or three exceptions none of the more 

 than a hundred papers having to do with the effect of tempera- 

 ture upon insects tell us anything about the effect of temperature 



i ' 4 Experimentelle Entomologische Studien vom physikalisch-ehemischen 

 Standpunkt aus. " Zweiter Band. Sophia, 1907. 



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