Seasonal Change in the Humidity Reaction of the Common Earwig, Forficula auricularia

Abstract

IN an investigation on physiological and ecological problems in Forficula auricularia, van Heerdt¹ found that well-fed, non-desiccated earwigs showed no humidity preference in the alternative chamber ; but if previously desiccated, the earwigs became distinctly hygropositive. Whenever Forficula is found in dry places, its presence there is, according to van Heerdt, to be ascribed to thigmotaxis. Experiments begun in the winter of 1946–47 gave results closely resembling those of van Heerdt. In the summer of 1949, however, when the experiments were continued, the earwigs (both specimens taken directly from very dry natural conditions, and specimens kept in moist air even for weeks) showed a clear and strong preference for the drier side of the alternative chamber. Two-hour experiments were carried out in the day-time, but in darkness with the exception of short intervals for counting. With a choice between 100–34, 100–97, 100–99.5, 97–87, 77–34, 34–20 and 34–0 per cent relative humidity, the drier humidity was consistently preferred by the summer earwigs. Desiccation, however, caused even the summer earwigs to choose the moist air. These same specimens, well-fed and kept in a moist atmosphere all the time, so that the effect of desiccation was excluded, were investigated in the following winter. It appeared that the animals no longer showed the dry reaction, but tended now to prefer 100 to 34 per cent relative humidity. The experiments were repeated in the summer of 1950 and winter of 1950–51, with results similar to those obtained in the earlier years.