Black Rove Beetle

2014 August 6

I found this rove beetle on July 25 while pushing my bike up the hill. It was longer than half an inch, fast-moving, and extremely slippery.

When it was alarmed, it would raise the rear of its abdomen up in the air, as if it were about to spray something noxious, but I didn’t notice any odd smells.

(or irritation to my skin, as far as that goes)

I think it is one of the three species in the genus Tasgius that have hitch-hiked over from Europe and become established in the Midwest. They look similar to the “Devil’s Coach Horse”[1], Ocypus olens, but those are bigger, meaner, smellier, and aren’t reported to have made it anywhere near Michigan yet. It is quite possible that this one didn’t even have any chemical defenses, and was simply pretending to be its more-dangerous relative. A tactic that probably works OK back in Europe and Asia, where they are both from. But probably not so well here in Michigan, where its relative has yet to put in an appearance.

It didn’t threaten me with its mandibles, so I couldn’t get a good shot of them, but from what we can see they appear to be long, scimitar-shaped things that would be good for grabbing soft-bodied prey, like slugs.

The forefeet have very large pads compared to the other feet. This may help it in hanging on to prey. It certainly doesn’t help it hang onto sticky surfaces, though – it was completely unable to climb the sides of the little ceramic bowl I had it in for a while.

These are predatory beetles, and spend a lot of their time under stones and in the leaf litter. So, if you find them in your garden, that’s OK. They are probably keeping down the slug population.

[1] The Devil gets all the coolest stuff. He also apparently has a lot of laundry to do.

3 Responses
  1. August 6, 2014

    I wonder how many abdomen behaviors they have. Is the activation of the abdomen a simple Boolean – 0 for idle, 1 for threaten?

  2. August 6, 2014

    Second thought, kind of like asking if the light goes out in the fridge when we close the door: I wonder what their false alarm rate is for abdomen-threatening. It would be an interesting detection theory exercise to find out how accurately they could discern threats from non-threats.

  3. August 6, 2014

    It actually had a couple of abdomen positions. The one I got a picture of was just elevated at about 30 degrees, but if I touched it the tail shot almost straight up for a few seconds. The height of the abdomen tip did seem to depend directly on how threatened it felt.

    And as far as distinguishing threats from non-threats, it didn’t take much provocation to make it raise its tail. Merely having my shadow pass across it was enough to get at least a small rise out of it. Since it didn’t seem to actually be spraying a chemical, there is essentially no cost to it to make a threat display when there is no threat, so I expect that they have false alarms all the time.

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