LAUSR

With increasing soil depth, the diel temperature amplitude gradually decreases and the phase of the temperature cycle is delayed. The onion fly Delia antiqua (Meigen) (Diptera: Anthomyiidae), which pupates at a soil depth of 2–20 cm, advances the eclosion phase of its circadian rhythm as the temperature amplitude decreases. This ‘temperature amplitude response’ is considered to allow pupae located at any depth to eclose in the early morning. The present study examines the temperature amplitude response of D. antiqua under thermoperiods with amplitudes ranging between 1 and 20 °C. The thermoperiod to which pupae are exposed is either a square‐wave temperature cycle, which is generally used for laboratory experiments, or a pseudo‐sinusoidal temperature cycle, which more closely resembles natural daily temperature cycle, at an average temperature of either 25 or 20 °C. Bursts of eclosion invoked by the abrupt temperature increase in square‐wave cycles are apparently repressed in pseudo‐sinusoidal temperature cycles. Eclosion time shows a clear quantitative shift by 5.9–8.6 h in response to temperature amplitude regardless of the average temperature and the shape of the temperature cycle. The change in the eclosion time is clearly noticeable between amplitudes of 1 and 8 °C but is small between amplitudes of 8 °C or more. Delia antiqua may evolve to compensate for the depth‐dependent phase delay of temperature changes within the possible range of temperature amplitudes in actual soil environments.