Factors in the selection of oviposition mode in Sympetrum infuscatum (Selys) (Anisoptera: Libellulidae)

The study was conducted at rice paddies in the cool temperate area of central Japan. The oviposition time period was limited to between ca 10;00 a.m. and 14:00 p.m. with a peak around noon. At an ambient temperature (Ta) below 30°C most pairs oviposited in tandem (TO) but at a Ta above 30°C in mid-summer most pairs separated shortly after the start of TO. The 9 9 continued to oviposit while being escorted by their partners. The regression coefficient of 3 body temperature (Tb) in ovipositing pairs was characteristically greater than that of the 9, and it is suggested that the 3 is more dependent on Ta than is the 9. Although the duration of oviposition was a little longer in pairs that separated after the start of oviposition, this difference was not significant. The reason why S. infuscatum starts oviposition in such a hot season of summer seems to be due to the morphological feature of its slender abdomen, which decreases abdominal light absorption at low Ta in the autumn.