湖山池におけるウチワヤンマ生息最終年の羽化消長

ウチワヤンマは湖山池でもっとも普通な大型のトンボだったが，2012年3月にはじまった塩分導入事業により姿を消した。ただし，塩分導入のはじまった直後の2012年6月から8月までは成虫の羽化が確認された。この羽化期間中に湖岸の脱皮殻を毎日回収し次のような結果を得た：1）雌のほうが早く羽化する傾向がある（雌先熟），2）体サイズ（後脚腿節長）は雌のほうが雄よりも，また，雌雄とも早く羽化した個体のほうが大きい。本種の雌先熟傾向は長野県諏訪湖でおこなわれた研究でも確認されているが，トンボを含め昆虫では珍しい。The golden frangetail Sinictinogomphus clavatus (Fabricius, 1775) (Odonata: Anisoptera: Gomphidae) was one of the commonest dragonflies in Lake Koyama in Tottori City (Tottori Prefecture, Honshu, Japan) before the intentional induction of higher salinity (3.5 – 8.75 ppt) in March in 2012 by the local government. We surveyed pattern of adult emergence of the species by collecting exuviae of larvae every day during the adult emergence from June to early August in 2012 which became the last year of the occurrence of the species in the lake. The results obtained were as follows: 1) There was a tendency that females tend to emerge earlier than males (protogeny). 2) Females were larger than males and individuals molted earlier were larger than those emerged later in both males and females. No adult emergence from the lake has been observed since 2013 due to high salinity.