According to Wikipedia, the Great Diving Beetle is a voracious predator, hunting insects, tadpoles and small fish in ponds. However, they sometimes come out and fly looking for a new pond, which they find by the reflection of the moon on the surface of the water.
This one was running about in the polytunnel where I was flint knapping. I think it was fooled by the flood lights we had on.
Information on Arkive suggests that this beetle is a male, as it has the large suction pads on its front legs and the wing case is not ridged.
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Great diving beetle
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