The selection for early and late pupariation in the flesh‐fly, Sarcophaga argyrostoma , and its effect on the incidence of pupal diapause

. Artificial selection from the first and last larvae of Sarcophaga argyrostoma (Robineau‐Desvoidy) to form puparia gives rise to two strains, ‘fast’ (F) and ‘slow’ (S), ‘fast’ pupariating about 3 days earlier than ‘slow’ in continuous light, 25°C. The two strains differ only in the time from larval wandering to pupariation; other aspects of development are identical. In light‐dark cycles (1°C) the distributions of pupariation times in ‘fast’ are unimodal and nearly normal, whereas those for ‘slow’ are multimodal and with a marked skew, especially in short daylengths. Pupariation times in ‘fast’בslow’ hybrids, an F 2 generation, and a backcross [(F×S)×S] are intermediate between ‘fast’ and ‘slow’ but incline towards ‘fast’. It is concluded that control of pupariation time is polygenic, and that ‘slow’ contain considerable residual variation. When compared with the unselected stock, ‘fast’ and ‘slow’ both produce a reduced incidence of pupal diapause in short daylengths, and ‘fast’ show a shorter critical daylength. These effects are interpreted in terms of a modified version of Gibbs' (1975) photoperiodic ‘counter’ hypothesis.