Change in Photoperiodic Sensitivity during Hibernation in a Semi-Aquatic Bug, Microvelia douglasi (Heteroptera : Veliidae)

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Abstract
Timing of induction of reproductive diapause of Microvelia douglasi were investigated. Diapausing females began to appear in early September in field population. Under laboratory condition at 24.degree.C, reproductive diapause was induced by photoperiods shorter than 12.5 hr of light, which coincides with the daylength in early September. Seasonal changes in the pattern of egg production of overwintering females incubated under laboratory conditions at 24.degree.C were also examined. When the insects were incubated under 16L-8D photoperiod, almost all the females began to lay eggs irrespective of the sampling date. When incubated under 12L-12D and 8L-16D, the percentage of females which laid eggs was low in samples collected before December, but it became as high as 100 percent in samples collected after December. The preoviposition period from the date of the collection increased until the end of October, decreased to 5 to 6 days in late December and stayed at a constant level thereafter. Although females incubated under 16L-8D continued to lay eggs throughout experimental period, some females, which were incubated under 12L-12D and 8L-16D, stopped laying eggs soon after beginning of incubation. However, under 12L-12D, the percentage of such females decreased in samples collected later in the winter.