1. Tenebrio molitor adults show a behaviour reaction to humidity. When normal undesiccated specimens are offered a range of humidities, they tend to collect in the drier region.
2. When the highest relative humidity available is not above 70% R.H., the reaction is slight but real. In a series of tests, as the highest humidity available approaches saturation, the intensity of reaction increases rapidly. It becomes nearly as intense as possible when the moistest region in the gradient approaches 100% R.H.
3. The difference between the two extreme humidities available--or the steepness of the gradient of humidity--is unimportant in determining the intensity of reaction. It is the value of highest humidity that matters.
4. The intense reaction shown when the upper humidity approaches saturation is very consistent; it reaches its maximum value after about 15 min.
5. The behaviour of these insects appears to be more closely correlated with relative humidity than with saturation deficiency.
Apart from preliminary experiments, all the experimental work was done by D. P. P.