Critical weight for the induction of pupariation in Drosophila melanogaster : genetic and environmental variation

The timing of maturation in insects is determined by reaching a critical developmental stage, at which development becomes committed to metamorphosis (e.g. Nijhout, 1981). The critical stage is a well-de®ned developmental stage, marked by a small ecdysone pulse (Berreur et al., 1979). It leads to a shift in hormonal levels, which sets the stage for further developmental changes (Riddiford, 1993). The critical stage occurs during the last larval stage, generally well before metamorphosis. Development after the critical stage is limited to a narrowly de®ned time period until the onset of pupation in holometabolous insects (Beadle et al., 1938; Bakker, 1959, 1961; Robertson, 1963; Nijhout & Williams, 1974) or adult ecdysis in hemimetabolous insects (Blakley & Goodner, 1978). The duration of this time period seems not to have been the subject of research. In a number of species, the critical stage is attained at a relatively constant size (Nijhout & Williams, 1974; Blakley & Goodner, 1978; Nijhout, 1979), indicating that the decision to enter the critical stage is based on some size-related character. Some species need an additional stimulus, like a minimal intake of nutrients (Wigglesworth, 1934; Nijhout, 1979; ZdaÂrek, 1985) or the presence of members of the opposite sex (Beck, 1971), before development will proceed. In Drosophila melanogaster, the critical stage occurs right after the second moult. After the critical stage, the developmental period is no longer affected by resource levels and can be completed in the absence of food (Beadle et al., 1938; Bakker, 1959; Robertson, 1963; Fourche, 1967). A reduction of growth rate before the critical stage increases the larval period, but does not affect adult size (Beadle et al., 1938; Robertson, 1963). This observation indicates that the critical stage coincides with the attainment of a relatively constant weight. The size of a larva at reaching the critical stage of