The present work was performed to clarify the photosensitive stage for wing-form determination and reproduction in Aquarius paludum. Only 40–60% of the adults were long winged when they were reared under 12 hr light-12 hr dark (12L-12D) during the first two instars and under 15.5L-8.5D during the rest of nymphal stage. However, all became long-winged, when they were exposed to 12L-12D until the end of the third instar. The exposure to 15.5L-8.5D till the end of the third instar induced short-winged adults of less than 15%, but long days until the fourth instar promoted about 40%. The exposure to long days either during the third, fourth or fifth instar promoted brachyptery, whereas all became long-winged when they were reared under long days either during the first or second instar. The exposure during the fifth instar induced maturing oocytes. The results indicate that the photosensitive stage for brachyptery occurs from the third to the beginning of the fifth instar, whereas that for oocyte maturation occurs during the fifth instar. Such a difference may refute the supposition of the oogenesis-flight syndrome that the two factors are tightly linked.