ABSTRACT. From a Japanese population of Chymomyza costata which has been known to have a photoperiodic larval diapause, we selected a mutant strain which did not respond to photoperiod. However, about 70% of the individuals of this strain entered diapause at 11oC irrespective of photoperiod, and about the same percentage of those of the photoperiod‐sensitive strain also did so in continuous illumination at 11oC. This indicates that low temperature induces diapause independently of photoperiod. On the other hand, a temperature drop from 18 or 25oC to 15oC and chilling at 4oC did not induce diapause.

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The genetic control of the photoperiodic larval diapause of Chymomyza costata was studied by hybridization, backcross and selection tests. The strains used in the experiment, normal photoperiodic‐diapause strains from Finland (VK) and Japan (SP) and a mutant non‐photoperiodic‐diapause strain (NP), which was selected from the Japanese population, differed from each other with respect to critical daylength and critical temperature measured at long‐day or short‐day conditions. The loss of photoperiodic response in NP was shown to be associated with the lowering of the short‐day critical temperature to the level of the long‐day critical temperature. A recessive allele at a single autosomal locus is responsible for the trait. The differences between VK and SP, or VK and NP, in the critical daylength are due to at least two loci, one on the X chromosome and one on an autosome. The critical daylengths of F1 females obtained from reciprocal crosses between SP and VK were intermediate between those of the parental strains. SP and VK responded quantitatively to photoperiod in diapause induction; a shorter photoperiod was more effective in inducing diapause than a longer photoperiod.