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Hydaticus pacificus conspersus Régimbart, 1899 (Coleoptera: Dytiscidae), is currently considered “endangered,” according to the national Red List of Japan. Herein, the developmental period of the immature stages of H. pacificus conspersus during laboratory rearing is described in detail for the first time. We found that this species deposits eggs not only on aquatic plants but also on the surface of driftwood. The observed durations of developmental stages were as follows: egg period – 3 days; first instar – 1–2 days (1.7 ± 0.5); second instar – 1–4 days (2.4 ± 0.6); third instar – 4–7 days (5.0 ± 0.7 days); and construction of a pupal chamber to emergence: 8–13 days (9.6 ± 0.8). Based on these observed results, H. pacificus conspersus has one of the shortest larval developmental periods among the Dytiscidae species. The short larval period and the ability to lay eggs on substrates other than plants may be an adaption of the species to increase reproductive success in unstable environments.

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