Procladius choreus (Meigen) emerge during the period from April to October in central Japan, in which air temperatures range from 12.0 to 32.0°C. Body size of adults decreased as the temperature exposure at immature stages increased. The smallest male, which was obtained in summer, was reduced to 62.2% of the largest one in wing length, and to 59.6% in females. Male genital structures also varied seasonally; gonocoxite was more slender and gonostylus had a longer posterior heel in spring and fall. Such tendencies in gonocoxite and gonostylus shape were related allometrically to male body size.