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The zygaenid moth Thyrassia penangae is a typical long-day species, entering winter diapause as a prepupa in a cocoon in response to short-day conditions during autumn. The previous photoperiodic responses under non-24-h light-dark cycle and Nanda-Hamner experiments (greatly extended dark periods coupled with a short photophase or a long photophase) revealed that the moth measured day length rather than night length in the determination of diapause. In this study, we further extend non-24-h light-dark cycle experiments and use the Bünsow experiment (the extended scotophase or photophase interrupted by 1-h light pulse or 1-h dark interruption) to test the photoperiodism for diapause induction of this moth. The extended non- 24-h light-dark cycle experiment showed that when the length of photophase exceeded critical day length, diapause incidence was quite low regardless of scotophase duration. No circadian rhythmicity was found in Bünsow experiments. The incidence of diapause was consistently low when preinterruption photophase was longer than the critical day length. The two experiments provide further evidence that this moth measures day length instead of night length. [ABSTRACT FROM AUTHOR]