ARAI Tetsuo

The egg period, egg size, and daily changes in egg size were investigated in the field cricket, Teleogryllus emma. T. emma inhabits the slope of the Oishi Dam (OD) (38.03°N, 139.57°E) and Arakawa riverside (AR) (38.09°N, 139.57°E) in Sekikawa Village, Niigata Prefecture, Japan. We studied OD, AR, and the F1 hybrids, viz, OdAr (OD females × AR males), and ArOd (AR females × OD males) and determined whether daily changes in egg size are related to shortening of the egg period of T. emma on the dam slope. The egg periods in OD, AR, OdAr, and ArOd were shorter at higher temperatures (p ≤ 0.05). Furthermore, the egg period in OD was shorter than that in AR and OdAr (p ≤ 0.05). After oviposition, the eggs enlarged due to water absorption and entered diapause at the embryonic stage of the array. The major axes of eggs in OD, AR, OdAr, and ArOd expanded daily and were influenced by temperature whereby a higher temperature resulted in an increase in the major axes. At 15ºC, the major axis gradually increased after oviposition, reaching its maximum at 130 days. The eggs grew rapidly at temperatures ≥ 20ºC and reached their maximum size at 10−14 days, 5−7 days, and 4−6 days after oviposition at 20, 25, and 30ºC, respectively. The expansion of the major axis up to 7 days after oviposition showed that the egg size in AR increased faster than that in OD at 15ºC, while that in OD increased faster than AR at 20, 25, and 30ºC, indicating that the expansion rate in OD and AR eggs differed with temperatures. Temperature-dependent changes in the expansion rates in OD and AR up to 7 days after oviposition suggest that they may be associated with shortened egg period.

The mechanisms involved in triggering hatching behavior in the katydid Eobiana engelhardti subtropica (Orthoptera: Tettigoniidae) remain largely unknown. Therefore, in this study, I aimed to clarify the mechanism of hatching behavior triggered by a light-on (L-on) signal. Hatching under transition from continuous darkness to continuous light (i.e., L-on step, dark pulses, and photoperiods) was concentrated within 2 h after the L-on, indicating that it was triggered by the L-on signal. Furthermore, hatching was directly triggered by a single L-on signal. However, the time from L-on to hatching under dark pulses and photoperiods was affected by the duration of the dark period. This dependency on the dark period’s duration before L-on indicates that it differs from the hatching trigger under the L-on step. The time from L-on to the earliest hatching time and the mean hatching time were similar under dark pulses and photoperiods. Those times were also similar at 15 and 20°C. This suggests that the time from L-on to hatching under dark pulses and photoperiods exhibits temperature compensatory properties, a characteristic of biological clocks. The time ranges from a few minutes to several tens of minutes; however, it is suggested that it is controlled by a time-measuring mechanism involving dark periods. Although it is well known that the L-on signal is involved in the time-measuring mechanism, to the best of my knowledge, this is the first study to report that the time-measuring mechanism is involved in the triggering of hatching behavior based on the L-on signal.

The onion fly, Delia antiqua (Diptera: Anthomyiidae) spends hot summers and cold winters in the pupal stage. Pupal diapause is determined by the temperature and photoperiod of the larval stage and the temperature of the pupal stage. When the larvae were reared under various photoperiods at 25°C and the pupae were kept at 25°C after pupation, the rate of diapause tended to be slightly lower in photoperiods longer than 13 h in the light period. When the larvae and pupae were kept at 20°C, the rate of diapause was 80% or more in photoperiods with a light period shorter than 13 h. However, the rate of diapause gradually decreased during longer photoperiods, and when the light periods were longer than 16 h, it decreased to approximately 25%. The larvae were reared under light-dark conditions (12 h light: 12 h darkness) (LD 12: 12), 14: 10, 18: 6, and 24: 0 at 30, 25, and 20°C and the pupae were kept in continuous darkness (DD) at 30, 25, 20, and 15°C. When the larvae were reared at 30 and 25°C and the pupae were kept at 25°C, the rate of diapause was 15% or less regardless of the photoperiod. When the pupae were kept at 30 and 20°C, the rate of diapause was 50−75% and almost 100% at 15°C. When the larvae were reared under LD 14: 10, 16: 8, and 24: 0 at 20°C and the pupae were kept at 25°C, the rate of diapause was 15% or less. The rate of diapause the pupal temperature of 30°C was 50−75%, at 20°C was 30−65%, and at 15°C was almost 100%. When the larvae were reared under LD 12: 12 at 20°C, the rate of diapause at pupal temperature of 30°C was 50−75%, at 25°C was approximately 55%, and it was almost 100% at 15°C. When the larvae were reared under LD 12: 12 at 20°C and the pupae were kept for 4 weeks under DD at 20°C after pupation. And the pupae were transferred from 20°C to 7°C and then kept for 10, and 15 weeks at 7°C and then transferred to 25°C. When the pupae were kept for 10 weeks at 7C, adult emergence was concentrated the 11th day (the 109th day after pupation) after transition to 25°C, and on the 10th day (the 143th day). This indicates that diapause was broken by low temperatures (7°C). When pupae were kept under DD at 20°C after pupation, adult emergence started around 50 days after pupation and continued for more than 50 days thereafter. No concentration of emergence was observed even when the temperature was changed to 7°C after the emergence adults. This indicates that diapause was broken at 20°C. The larvae were reared under continuous light at 15°C and the pupae were kept under DD at 7°C after pupation and transferred to 25°C. When the pupae were kept at 7°C for 1 to 20 weeks, the longer the period of 7°C, the shorter the period from the transition to 25°C adult emergence. Adult emergence was concentrated when the 7°C period was 11 weeks or more, and the tendency was more pronounced as the 7°C period increased. When the larvae were reared under LD 12: 12 at 30 and 25°C and the pupae were kept under DD at 30, 25, 20, and 15°C, the rates of diapause were compared in 1981 and 2005. When the rates of diapause when the larvae were reared at 20°C, was compared between 1981, 2005, and 2006. When the larvae were reared at 30 and 25°C, the rate of diapause was lower in 1981 than in 2005, regardless of the temperature of the pupae. However, when the larvae were reared at 20°C, the rate of diapause was higher in 1981 than in 2005 and 2006, regardless of pupal temperature. These results indicated that many years of successive rearing influenced pupal diapause decisions. The most interesting finding was that in many years of successive rearing, the selective pressure for diapause changed with the rearing temperature of the larvae. This elucidation will be clarified by through genetic analysis in the future.