Picture this

 

You're a beach hopper, foraging on kelp washed up on the beach.  It's nighttime, so it's dark, and out of nowhere, you're seized from behind by a giant pair of sickled-shaped mandibles.  You've fallen prey to a Pictured Rove Beetle (Thinopinus pictus).

Here's a close-up of a similar scene:

 

In the previous photo, the Pictured Rove Beetle is holding onto the beach hopper with its curved brown mandibles.  Although it's a little confusing because they blend together, the beach hopper is recognizable by its blue antennae and legs.

Pictured Rove Beetles live on sandy beaches along the Pacific Coast.  They burrow below the sand during the day, then emerge at night to hunt for beach hoppers and other prey.  They prefer wet sand and move up and down the beach with spring and neap tides.

This is the same beetle viewed from above.  Note that these adult beetles are wingless.

 

While we were watching this beetle on Salmon Creek Beach, a second individual emerged with a slightly different color pattern: 

 

If you're wondering, Pictured Rove Beetles are ~2 cm long...not so large that you have to worry about them mistaking one of your toes for a beach hopper!

P.S.  Many thanks to Eric for spotting these beetles and to Peter Connors for first introducing me to these voracious burrowing predators.