A ground beetle showing off its massive mandibles. Aurora, Nebraska
I found this ground beetle in my yard when I was pulling up some bricks. It’s a very common, though not often seen, beetle around here – and its close relatives are abundant throughout much of the United States. This one appears to be Scarites vicinus, though it’s difficult to be sure.
This Carabid beetle spends its days beneath the ground, burrowing in moist soil or beneath logs (or bricks!). It comes out at night to hunt on the surface, killing and eating just about any small invertebrate it encounters.
If it is spotted by a mouse or other potential predator, the beetle’s first defense is often to “play dead” (see below) but if that doesn’t work, a pinch from its impressive mandibles can be a good fallback option!
When threatened, the beetle pulls in its vulnerable legs and antennae and stiffens up, looking for all the world like a dead beetle.
Many thanks to Ted MacRae for his help with identification and supplemental information on this beetle. Visit his blog at: http://beetlesinthebush.wordpress.com/
The Prairie Ecologist 
Those mandibles look painful! How big is that beetle?
About an inch long. Yes, I would imagine they are pretty painful!
Rumor has it these guys use those mandibles to dispatch cutworms and the like. Looks like pretty good equipment for that (with the biting of fingers only an incidental use). When I was a kid, I had a pet skunk that just loved to pounce on these and eat them – hold down the head and crunch into them from the rear. I always wondered how it could stand the nasty, staining, defensive chemicals secreted from the abdomen???