(31(4):334-338)Biological Studies on Microvelia douglasi, a Predator of the Brown Planthopper

道氏廣肩水黽（Microvelia douglasi Scott ）具有翅與無翅兩型。水稻插秧後2 週左右，該蟲即在稻田出現。成蟲、若蟲多棲息在靜止水面。兩型成蟲能互行交尾，交尾後24 小時內即在水際產卵，兩型雌蟲之每雌產卵數平均為20~25 粒。卵期4~6 天，孵化率平均約66.3% 若蟲有5 齡，若蟲期8~14 天；在常溫下完成1 代約需16 天。成蟲壽命平均為25~37 天。田間雌雄性比為1.45 : 1 。 成蟲與若蟲均為捕食性，食餌廣泛包括稻田常見昆蟲與蜘蛛等，對大型食餌常成群圍攻刺吸，無明顯之自相殘殺行為，其對稻褐飛蝨之捕食以3 齡以下之若蟲居多。該蟲在稻田之棲群可以在褐飛蝨發生之前建立，但受田間排水之影響極大。本文記述其各期形態特徵與生活習性。 The broad-shouldered water-strider (Microvelia douglasi Scott, Veliidae, Hemiptera) has been recognized as one of the predators of the brown planthopper (Nilaparvata lugens). Observations were made on both winged and wingless forms of the water- strider. It should be noted that mating occurred between individuals, regardless of form. Reproductive habits were recorded as follows : At room temperature, each female laid an average of 20-25 eggs on the water surface, usually within 24 hours of mating. Approximately 66.3% hatched, after an incubation period of 4-6 days. During the 8-14 days nymphal stage, the nymph moulted 5 times, and it took 16 days to reach the adult stage. The longevity of adults was from 25-37 days. In the rice fields the sexual ratio was found to be 1.45♀: 1♂ . Nymph and adult of broad-shouldered water striders were found walking rapidly on the water surface of paddy fields. They preyed upon both adults and nymphs of the brown planthopper, as well as other insects which had fallen onto the water. The 3rd or younger instar nymphs of the brown planthopper seemed to be preferred. The water- striders most commonly attacked a single prey all together immediately after finding it. Cannibalism, even in adverse conditions, was rarely found. The douglasi water-strider inhabits any water surface, including drainage ditches, marshes, etc. They usually appear in the rice paddy fields about 2 weeks after the rice has been transplanted. Their population builds up before the occurrence of the brown planthopper, and apparently declines as the water in the paddies drains