Platambus ussuriensis, (Nilsson, 1997)

Watanabe, Kohei, 2022, Biological Notes on Immature Stages of Platambus ussuriensis (Nilsson, 1997) and Copelatus nakamurai Guéorguiev, 1970 (Coleoptera: Dytiscidae), The Coleopterists Bulletin 76 (2), pp. 233-236 : 233-235

PLATAMBUS USSURIENSIS ( Nilsson, 1997)

Platambus ussuriensis ( Dytiscidae ) has been recorded in Japan, Russia, China, and Korea ( Miyake 2020; Nilsson 1997). In field surveys in Japan, adults are found from late February to early November and are reported to disappear during winter ( Sakai 2021). Eggs and larvae have not been discovered ( Sakai 2021), and there is no information on the immature stages.

Ten P. ussuriensis adults ( Fig. 1A ) were collected from Tsushima Island, Nagasaki Prefecture, Japan, on 23 February 2021. They were brought to the Ishikawa Insect Museum (Hakusan-shi, Ishikawa Prefecture) on 27 February 2021. Two male and three female adults were reared in a plastic rearing cup (13 cm × 6 cm × 3 cm, diameter × height × water depth). Two fallen leaves and one small branch were placed in these cups to serve as hiding and oviposition sites for adults; the adults were fed with frozen chironomid ( Diptera : Chironomidae ) larvae at two- to three-day intervals. The rearing cups were checked daily for the presence of eggs. The detection date and developmental stage of the eggs were recorded. The eggs were transferred into individual plastic “larval cups” (8 × 4 cm, diameter × height) with approximately 5 mm of water.After the larvae hatched, they were provided daily with living chironomid larvae. For pupation, covered plastic “pupation cups” (equal in size to the larval cups) were prepared with crushed and moistened peat moss (2-cm depth) as a landing substrate. Individual third-instar larvae were carefully placed in the pupation cups when they stopped consuming prey, moved continuously, and their midgut content was not visible. Hatching, molting, transition to landing substrate, and emergence dates of the new adults on the soil surface were recorded every 24 h.All cups were kept at 19 °C under a 12:12 h L:D cycle in an incubator (Biotron LPH-120S, Nippon Medical & Chemical Instruments Co., Ltd., Osaka, Japan). Photographs were captured using a Nikon D500 digital camera (Nikon, Japan), equipped with a Laowa ULTRA MACRO 25 mm f/2.8 2.5–5× lens (Laowa, China) ( Figs. 1B, D–E ) and a Nikon AF-S VR MICRO-NIKKOR 105 mm f/2.8G ED lens (Nikon, Japan) ( Figs. 1A, C, F–G ).

This is the first study to report the egg and larval stages of P. ussuriensis . The first eggs of P.ussuriensis were found on 5 March 2021, six days after adult rearing. All eggs were laid on the surface of a dead leaf submerged in the water ( Figs. 1B–C ). The egg period lasted 6–13 days (mean ± SD = 10.7 ± 3.3 days, n = 3). The first- ( Figs. 1D–E ), second- ( Fig. 1F ), and third-instar ( Fig. 1G ) P. ussuriensis larvae lasted 2–8 days (5.5 ± 2.5 days, n = 2), 10 days (n = 1), and 8 days (n = 1), respectively. The total larval development period lasted 21 days (n = 1). All the larvae were fed live chironomid larvae ( Fig. 1E ) from the day of hatching.After transitioning to land, the larva dug into the soil and constructed a pupal chamber. The duration of the prepupal and pupal stages could not be determined because the inside of the pupal chamber could not be observed. The duration from landing to escape was 23 days (n = 1).

In field surveys in Japan, many teneral adults were found during late June ( Sakai 2021). Based on the 50- day period observed for the complete development of P. ussuriensis (n = 1) in the present study, their reproductive season in Japanese fields is estimated to be spring. The number of eggs laid was low during this rearing, which could be because the temperature or day length conditions were inappropriate.

Morphological aspects of the larvae were confirmed via photographs ( Figs. 1D–G ). An occipital suture was observed on the head of the second- and third-instar larvae; however, the presence of an occipital suture on the head of the first instar was not determined. One of the known characteristics of Agabinae larvae, including those of P. ussuriensis , is the absence of occipital sutures (Alarie and Michat 2020). All larval instars of Platambus pictipennis (Sharp, 1873) , Platambus convexus Okada, 2011 , and Platambus fimbriatus Sharp, 1884 belonging to the P. maculatus species group, as well as Platambus sawadai (Kamiya, 1932) of the P.sawadai species group ( Okada et al. 2019), have no occipital suture on the head. Conversely, the second- and third-instar larvae of Platambus koreanus ( Nilsson, 1997) , which belongs to the same P. optatus species group as P. ussuriensis , has an occipital suture ( Nilsson 1997). Photographs of Platambus optatus (Sharp, 1884) and Platambus stygius (Régimbart, 1899) , from the P. optatus species group, in a study by Hayashi (2015), were also confirmed to have an occipital suture. An occipital suture may be one of the characteristics common to all species of the P. optatus species group. Further studies, including those on other species whose larvae have not been identified, are required to confirm whether the group’s occipital suture is a unique trait.

Figures

Fig. 1. Platambus ussuriensis (A–G) and Copelatus nakamurai (H–K). A) Adult, B–C) Egg laid on dead leaf, D, H) First-instar larva, E) First-instar larva preying on a living chironomid larva, F, I) Second-instar larva, G, J) Third-instar larva, K) Second-instar larva preying on a living chironomid larva. Scale bars: 1.0 mm.