The response of the blowfly Phormia regina to stimulation by alcohols in the vapor phase has been investigated by means of an olfactometer which permitted quantitative control of stimulus concentration. The median rejection threshold was selected as a criterion of response. As was true in the case of contact chemoreception, the distribution of thresholds in the fly population is normal with respect to the logarithm of concentration. In terms of molar concentration the alcohols are rejected at logarithmically decreasing concentration as chain length is increased. Beyond decanol there is no further stimulation. When thresholds are expressed as pressures and plotted against saturated vapor pressures on logarithmic coordinates, the data fit a line the slope of which is not significantly different from 1; i.e., the thresholds vary directly with vapor pressure. Individual threshold values, however, deviate significantly from this line. and the deviation must be ascribed to other factors which have not as yet been identified. When thresholds are expressed as activities, all alcohols are equally stimulating. It appears that the limiting process of olfaction, at least in so far as the normal alcohols are concerned, may involve an equilibrium process. Conformity to this concept is most exact for intermediate members of the series.

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