1. Ptinus tectus adults react to a humidity gradient by collecting in the drier region.
2. The reaction to a given difference of humidity is most intense at low humidities; it declines to nothing at about 75-90% R.H. and increases again at 90-100% R.H. No such variation has previously been found.
3. A kinetic mechanism of reaction is involved. The higher the humidity the higher the locomotory activity over a period of a week.
4. Desiccated animals are more active than normal ones. At low humidities they show a much weakened reaction towards drier air; at high humidities the normal reaction is reversed and the animals collect in the wetter region.
5. Desiccated animals given water to drink immediately begin to behave like normal ones.
6. Some of the humidity receptors appear to be located on the antennae. Removal of the distal joint increases the intensity of reaction in high humidities, but the interpretation of this is questionable. The reaction remains strong when up to five joints are amputated and then it declines. When only one or more of the last three joints remain, a reversed reaction occurs, similar to that shown by desiccated individuals.
7. This suggests that there are two kinds of humidity receptors, the first kind being mainly located distally on the antennae and some of the second kind being either on the basal joint or not on the antennae at all.
8. Attempts to identify humidity receptors were not successful.