A rare diving beetle in Rye

26th February 2012, Sunday

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On Thursday morning I found this large water beetle in my moth trap in Rye, length 36 mm. Although similar to the common Great Diving Beetle (Dytiscus marginalis) illustrated in Chinery’s Collins Pocket Guide to Insects, there were notable differences, so I searched for Dytiscus beetles on the web and found that it matched a female D. dimidiatus, the Thick-horned Diving Beetle. In March 2000 the Interreg II Project published a major report, The Coleoptera of Rye Bay, by Barry Yates and P.J. Hodge. This said that Dytiscus dimidiatus is confined to a few areas of ancient wetland including the Somerset and Gwent Levels, Wicken and Woodwalton Fens, the Norfolk Broads, Romney Marsh and Rye Bay. In Britain it is now classified as Near Threatened. In Rye Bay the first record was at Northpoint in 1950 and there were only four more up to 2000, but Brian Banks tells me that he now finds it annually at Dungeness, New Romney and East Guldeford, so it seems to be spreading.

Dytiscus dimidiatus is not included in Chinery’s book but is similar to marginalis except for the lack of orange edges to the prothorax. I gave the beetle to Barry Yates, whose son Lewis took the four photos shown here, emailed to Peter Hodge for confirmation. While in Barry’s charge, the beetle laid an egg measuring 6.5 mm, shown here in the final photo. Even in February, running a moth trap can turn up surprises – especially in Rye!

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