Copelatus nakamurai, GUEorgUieV, 1970

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 : 235

COpELATUS NAKAMURAI GUÉorgUieV, 1970

Copelatus nakamurai ( Dytiscidae ) is endemic to Japan and listed as “Vulnerable” in the Red List of Japan (Ministry of the Environment of Japan 2020). It is the only member of the C. chevrolati species group in Japan. Tajima and Yanagida (2010) report- ed the basic life history of this species. The firstinstar larvae of C. nakamurai are illustrated for the first time in the present study.

Three C. nakamurai adults were collected from Gif Prefecture, Japan, on 4 May 2018. They were brought to the rearing room at the Ishikawa Insect Museum, maintained at 26 °C under a 9:15 h L:D cycle, on 6 May 2018. The rearing method described by Watanabe et al. (2017) and Watanabe and Hayashi (2019) was used. Photographs were captured using the same equipment as for P. ussuriensis .

The first eggs of C. nakamurai were found on 8 May 2018, two days after adult rearing began. The eggs were laid singly by adults and covered with a viscous substance, enabling the eggs to adhere to the surface of Java moss, as illustrated by Tajima and Yanagida (2010). The egg stage lasted 2–3 days (2.7 ± 0.5 days, n = 10). The first- ( Fig. 1H ), second- ( Figs. 1I, K ), and third-instar ( Fig. 1J ) larvae lasted 2–5 days (2.8 ± 0.9 days, n = 10), 3–8 days (4.1 ± 1.4 days, n = 10), and 9–11 days (10.3 ± 0.6 days, n = 10), respectively. The total larval development stage lasted 15–23 days (17.2 ± 2.2 days, n = 10). From the day of hatching, all C. nakamurai larvae swallowed whole live chironomid larvae and members of the suborder Tubificina ( Fig. 1K ). The same predatory behavior was observed in Copelatus parallelus Zimmermann, 1920 and Copelatus masculinus Régimbart, 1899 ( Watanabe et al. 2017; Watanabe and Hayashi 2019). This predatory behavior is possibly a characteristic of Copelatinae larvae as described by Balke and Hendrich (2016) and Miller and Bergsten (2016).

After transitioning to land, the larvae dug into the soil and constructed a pupal chamber. The duration of the prepupal and pupal stages could not be determined because it was not possible to observe the events inside the pupal chamber. Only one new adult escaped from the pupal chamber, and the duration from landing to escape was 40 days (n = 1). The remaining nine individuals did not escape from the soil. By digging into the soil, it was possible to confirm that six new adults died in their pupal chambers without coloration, and two live new adults remained in their chambers without coloration. The other individual was not found in the soil. The duration from larval landing to the excavation of two live new adults was 39 and 42 days, and the duration from larval landing to excavation stage of six dead new adults was 50 (n = 2), 61, 63, 68, and 75 days.

A summary of the life history and habitats of this species has been reported by Tajima and Yanagida (2010). The developmental period of C. nakamurai reared at 26 °C was similar to that reported by Tajima and Yanagida (2010) at 24–28 °C and 16:8 h L:D (egg, 2–3 days; first instar, 2–4 days; second instar, 3–5 days; third instar, 7–15 days). Tajima and Yanagida (2010) also reported that the larvae of C. nakamurai spent 7–17 days in transition from the landing to the prepupal stage, and the pupal period was 4–5 days. Conversely, Tajima and Yanagida (2010) described that new adults became colored and escaped from the pupal chamber one day after emergence. However, in the present study, only one new adult escaped from the pupal chamber, and the other eight individuals were not colored and remained in the pupal chamber for more than a month. The photo by Tajima and Yanagida (2010) shows a new adult found on tissue paper with the exuvia of the third instar larva, and its body was not colored. Furthermore, a detailed description of the prepupal and pupal periods suggests that the pupal chamber was artificially destroyed to observe the individuals. Watanabe et al. (2017) suggested that C. parallelus has a life cycle in which new adults do not escape from the pupal chamber on their own. The results of the present study suggest that most of the new adults of C. nakamurai , as well as of C. parallelus , do not escape from the pupal chamber on their own, and they escape during rainfall or when the water level rises. In this study, the new adults of C. nakamurai in the pupal chamber were alive for 39–42 days after landing. However, after 50–75 days, the new adults were confirmed dead. Since new adults of C. parallelus were reported to survive in the pupal chamber 68 days after emergence ( Watanabe et al. 2017), it is possible that the new adults of C. nakamurai are less desiccation-tolerant than those of C. parallelus . Tajima and Yanagida (2010) estimated that the reproductive season of C. nakamurai is from June to August. In the present study, the adults collected on 4 May 2018 laid eggs four days later; thus, egg laying is estimated to begin during May in the field.

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.