The cricket Modicogryllus siamensis Chopard shows photoperiod-dependent changes in the duration of nymphal development: nymphs become adult within 60 days after hatching, undergoing seven moults under long-day conditions, whereas, under short-day conditions, nymphal development takes much longer (approximately 180 days) with an increased number of moults. Because removal of the compound eyes alters this photoperiodic response, the eyes may be involved in light detection during the photoperiodic response. The role of opsins, expressed in the compound eye, is examined in the present study with reference to the photoperiodic response. Molecular cloning identifies cDNAs of three opsins, opsin-Ultra Violet (Ms'op-UV), opsin-Blue (Ms'op-B) and opsin-Long Wave (Ms'op-LW), and in situ hybridization reveals that the opsin genes are expressed in specific regions of the compound eye in a gene-specific manner. RNA interference (RNAi) technology using the opsin genes results in a partial disruption in the long-day responses; most of the treated crickets showed eight or more moults and up to 23.5% show a prolonged nymphal period that is typical of short-day responses. Under short-day conditions, op-UV RNAi crickets show earlier adult development, whereas no distinct alterations are observed in op-B and op-LW RNAi insects. The results suggest that the opsin genes may play differential roles in the photoperiodic response in the cricket and that the results can be at least partially explained in terms of the external coincidence model of photoperiodic time measurement.