Studies in Diurnal Rhythms

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Abstract
The species of woodlouse investigated are known to stand in the following order as regards the rate of water loss by transpiration: Philoscia muscorum > Oniscus asellus > Porcellio scaber > Armadillidium vulgare. The intensity of the responses of the four species to humidity at room temperature (18±2° C.) are compared and are shown to stand in the same order. The intensity increases as desiccation proceeds. At low temperatures (3±2° C.) the intensities of the reactions to humidity are much reduced and are the same in all species; at higher temperatures (30±2° C.) the humidity responses of Ph. muscorum and O. asellus are again somewhat reduced, and it is suggested that this may be correlated with a lower thermal death-point and the need to effect a reduction in body temperature by evaporation. The antennal cone sensillae are shown to be contact chemo-receptors; no specific sense organs are concerned with the appreciation of humidity. The response is correlated with saturation deficiency rather than with relative humidity and is probably engendered by dehydration. Ph. muscorum is the most strongly photo-negative of the species and there is a graduation through O. asellus and P. scaber to A. vulgare which is the least so. Ph. muscorum is also shown to be the most intensely nocturnal in habit, A. vulgare the least. It is therefore suggested that the degree of nocturnal activity is correlated with the ability to withstand water loss by transpiration. The results obtained are discussed in relation to the ecology of the species.