Hybrid phenotypes before and after antagonistic selection. Photoperiodic response (a, c, e) and response to the NH protocol (b, d, f). (a, b) Before selection; (c, d) after antagonistic selection for long CPP and high amplitude (Up); (e, f) after antagonistic selection for short CPP and low amplitude (Down). CPPs (h) determined from logits; amplitudes of the rhythm from non-linear regression (% pupation). T=the total duration of light plus dark for each cycle. In all cases, the regression coefficients for the determination of critical photoperiod were significantly greater than zero (P⩽0.001). In all cases both the amplitude (P<0.003) and the period (P<0.001) were significantly non-zero, that is, NH always elicited a rhythmic response. Sample sizes±s.d. before selection averaged 92±7 larvae per day length for CPPs and 92±9 larvae per T-cycle for NH; sample sizes±s.d. after selection averaged 94±18 larvae per day length for CPPs and 86±12 larvae per T-cycle for NH. Data are provided in Supplementary Tables S1–S7. Symbols distinguish among ○ East, • West and ◊ North lines.