Beetle Serial Killers: Promecognathus

This is Promecognathus crassus, a carabid beetle with one purpose in life: wrecking millipedes. Day in, day out, Promecognathus roams the forest floors of the western US in search of helpless millipedes, murders them, devours the remains, calmly reflects on the fragility of life, and repeats the process.

Millipedes are known for being nature’s chemical anarchists, producing many noxious compounds to ward off predators. These substances include alkaloids, cardenolides, quinones, acids, and hydrogen cyanide. Scary as this cocktail sounds, millipedes are generally peace-loving creatures, and as far as I know, no person has ever died from interacting with one (though they might stain your fingers if you’re reckless).

Consider yourself lucky.

However, if you were to, for whatever reason, eat nothing but millipedes for the rest of your life (fad diets, am I right?!), you would likely beg for death’s sweet release.

So how does Promecognathus manage to gorge itself on millipedes despite all those toxic chemicals? Surely these beetles have evolved a tough digestive system or some enhanced metabolic pathways to detoxify their prey, right?

Turns out the answer is no. Hydrogen cyanide and quinones will liquefy the insides of a Promecognathus, same as the rest of us. The beetle’s secret to avoid an agonizing death is all about technique. And those jaws.


Promecognathus sneaks up on the millipede and delivers a swift bite between the poor critter’s protective carapace, severing its nerve cord and rendering it paralyzed. The incapacitated millipede is unable to protect itself, and Promecognathus has its wicked way. I assume this behavior was the inspiration for the best Jet Li movie of 2001.

Why does the beetle bother going through so much trouble for a meal?  Millipedes presumably taste bad even without the toxins. Simple: millipedes are big, really abundant, and nobody else is eating them. Every time a clever and seemingly impenetrable defense evolves, some  creature will find a goofy way to exploit it.

So next time you’re hiking in the Pacific Northwest, you may just encounter a Promecognathus on the prowl, so keep an eye on the ground.

And watch your back.

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