Temperature-dependent phototaxis in overwintering adults of the grasshopper Patanga japonica (Orthoptera, Acrididae)

In central Japan, adult Patanga japonica (Bolívar) grasshoppers overwinter as adults while in reproductive diapause. At this local, February nights fall as low as -7°C, whereas days can exceed 16°C. Adults respond to the diel thermal cycle with daily vertical movements out of and back into leaf litter. This paper documents and discusses the significance of this interesting winter behavior. Temperature strongly influenced the daily vertical movements. Time of morning emergence, duration of aboveground occupancy, and number of adults emerging all highly correlated with current and maximum daily temperatures. In January, adults were immobile at < -1°C but could stand up when their body temperatures reached ~3.7°C. In contrast, adults held outdoors in semi-natural conditions emerged from the litter at ~14°C, suggesting threshold temperatures of ~14°C for morning emergence. The numbers of adults emerging or hiding varied over the winter season. Light also influenced movements. Adults held in horizontal transparent tubes, each with half covered with black paper (D-area) and the other half exposed to light (L-area), moved into the L-area during the day and returned to the D-area in the afternoon. In both cases, movement was into a colder microhabitat, implying that the direction of daily movements was possibly via phototaxis, not thermotaxis. Further experiments suggested that increasing temperatures elicited positive phototaxis, and decreasing temperatures elicited negative phototaxis and that the phototaxis was controlled by the direction, magnitude, and absolute range of the temperature change in P. japonica.