Predaceous Diving Beetle

2012 May 16

Here’s another one that I photographed in my office on July 26, 2011. But, it was actually found dead in the back window of a car some months earlier, sometime around February[1]. And who knows how long it might have been there, it was completely dried out and had probably been rattling around since at least the previous summer.

This shows the big hazard of actual dried insect specimens: they are crazily fragile, and the legs break off at the slightest provocation. All that stayed on with this one are the two big swimming legs.

And I say “swimming legs” because this is clearly a Predaceous Diving Beetle in the family Dytiscidae. The long hairs on the strong hind legs make them very effective swimming paddles.

Even though they are predatory, they don’t have the huge mandibles that we normally associate with carnivorous beetles (unlike their highly-aggressive larvae) . It’s a shame that the front legs are missing, I expect that if they were still present we’d see that they were fairly strong and adapted for grasping prey.

Unlike the water scavenger beetle in the previous posting, this one has no keel and is thinner. I think this one’s hunting style is to lurk on the surface (with its brown mottling disguising it as a small dead leaf), with its downward-looking eyes used to spot likely prey items swimming underneath it. It would then quickly swim down to snatch it with a few strokes of its big, oar-like rear legs, which are good for a short burst of speed but not so much for sustained swimming.

[1] It was somebody else’s car, we were sharing a ride and I happened to be riding in the back, and so spotted the beetle lying there on the ledge under the rear window. It was intact when I spotted it, but then when I picked it up, four of the legs fell off.

One Response
  1. May 17, 2012

    If the hairs are sufficiently stiff, then there’s probably not much difference between that design and one with chitinous paddles on the ends of their legs. It would be a great graduate thesis project in fluid dynamics to examine that hypothesis.

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