A Great Diving Beetle?

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Posted by RustonHouse RustonHouse
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After the recent rain there four inches or so of water in the bottom of the small boat I use for lake maintenance. When I went to bail it out I discovered a large beetle swimming around in it.

Colymbetes Fuscus?
The creature was probably the biggest beetle I had ever seen, around 35mm long, and was clearly fully at home in the water as I could only think that it had come on board swept up in the weed I had been clearing from the pond earlier in the week.

Colymbetes Fuscus?

Colymbetes Fuscus?
As I bailed and the water slopped around in the boat it did seem to want to climb out of the way of the surging waves that raced up and down the boat, but once engulfed would happily swim. In the water it's main legs could be seen to have feathering on them that clearly were helpful for both propulsion and steering.

The closest I could get to identifying the breed on iSpot was Colymbetes Fuscus, but unusually, no one has responded confirming my identification.

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Re: A Great Diving Beetle?

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Today, I showed the photographs of the beetle to my to recently met friend, Tim. He knows a thing or two about aquatic plants and wildlife and his immediate re-action was "Great Diving Beetle". I have looked that up and discovered that the Latin name for that creature is Dytiscus marginalis. Looking at pictures of them, the one below is typical, it appears they are rather different. Not only do many photographs, such as the one below, show it with at least a green tinge, sometimes very distinctly green, they also have a very much clearer pale edges to all the various "plates" on their bodies. While there is some signs of a light strip around the edge of the body on my example there are no such stripes across the body on the plate behind the head.

Dytiscus marginalis
Discovering that both varieties fly was a revelation too. It hadn't occurred to me that such a monster could fly. It was also interesting that Colymbetes fuscus has no English name, so perhaps for the casual observer it is legitimate to call any such similar large beast "Great Diving Beetle", because they are certainly both great and dive!
Greg Chapman
Ruston House