The Pinacate Beetle is Not Afraid of You

Eleodes beetle | Photo: Chris Clarke

Taking my usual walk out in the open desert I saw it crossing the trail ahead: a low black dot with an air of confidence. It made no attempt to hurry for cover despite standing out plainly against the light gravel.

That's unusual for small desert animals. Almost everyone out here is on the make, looking for bits of food and moisture, and a beetle the size of a raspberry would ordinarily draw lots of attention from passing birds, lizards, and mice.

This beetle didn't care. It took its time, walking in a casual straight line from point A to point B, until I caught up with it. It turned to face my boot, calm as you please.

It was the placid self-assuredness of an animal that knew it could make life miserable for anything that bothered it.

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It was a member of genus Eleodes, one of the vast tribe of darkling beetles that scavenges the deserts of the western U.S. and Mexico. It was probably a member of the species Eleodes armata, though the light was low and my close vision poor without correction, and I am no beetle expert even when I can see clearly. I coaxed the beetle onto a stick and brought it near my face for closer inspection. Each of its little legs had tiny bristly spines, which tentatively confirmed my guess that it was an Eleodes armata.

A Pinacate beetle, then, strolling carefree through the desert. The Pinacate beetle has less to fear from predators than many other small desert animals. They must run and hide and use protective coloration, but the Pinacate beetle has a trick -- I almost said "up its sleeve," but that's not where he keeps it.

I'm probably imputing too much mammalian emotion to the beetle, ascribing states like "carefree" and "courageous" to it. The degree to which insects consciously apprehend the world is a matter of ongoing debate among scientists. But I can say this with some certainty: the Pinacate beetle didn't consider me to be too big a threat, even as I picked it up and peered at it.

It's pretty easy to tell when a Pinacate beetle considers you a threat. Confronted with something it doesn't like, the beetle will turn its back and do what amounts to a headstand.

That headstand is generally enough for any animal with sense to beat a hasty retreat. If not, the beetle will then let fly with a noxious spray of repellent chemicals from its hind end. Those chemicals are secreted and stored in special organs called "repugnatorial glands," a delightful technical term.

When they're not acting as the skunks of the beetle world, Pinacate beetles make their living by eating decaying organic matter, for definitions of "decaying" that include just-fallen flowers. They're active year-round, though they do hide from the hottest part of the day in the hottest part of the year. A typical rodent burrow might house several dozen hunkering Pinacate beetles at mid-day, each literally chilling out until the atmosphere cools and they can resume their search for tiny dead things to eat.

Ranging across most ot the southwestern U.S. from Oregon to Texas and well into Mexico, Pinacate beetles seem ubiquitous in habitat that suits them -- usually open desert and scrubland, dry washes, and sky island forests. The name Pinacate is of uncertain origin, with some sources translating it as a Mexican Spanish word for "clown" or "acrobat" as a reference to the headstand. More likely, the name comes from the Nahuatl word "pinacatl," which means "small black beetle," making "Pinacate beetle" a "the La Brea Tar Pits"-style bilingual redundancy.

However they got the name, the beetles lent it to a chain of desert volcanoes at the head of the Sea of Cortez that just barely stretches across the U.S. border west of Organ Pipe National Monument. The beetles are abundant there, and "Sierra Pinacate" sounds way better than "Stink Beetle Mountains."

I find Pinacate beetles engaging and pleasant, and for that reason in decades of hanging out with them in the desert I have never found the motivation to bother one enough to provoke it into action. So I can't give you a first-hand report on how that spray smells. Second-hand reports I've seen are not much help, using vague terms like "sharp" and "foul." Some mention that the smell cannot be washed off, a further disincentive to my experimentation.

The Pinacate beetle can spray its chemical weapon several times its body length, enough to dissuade most small predators. There are more than 30 known Eleodes species, with that number certain to increase as more coleopterists work on them. Each species carries this concealed weapon, though their method of delivery varies. Some have a greater range than the Pinacate beetle, while others merely dribble the substance all over themselves when threatened. This survival strategy is what gives the gigantic Eleodes tribe its other common name: "desert stink bug."

The precise mix of chemicals in the stink varies from species to species, but all of them seem to be variations on a class of chemical called quinones. Many insects secrete quinones, but most that do so use the chemicals in small doses as hardening agents for the chitin in their exoskeletons. If you're familiar enough with cockroaches to have noted that their armor gets darker and harder as they grow, you've seen "quinone tanning" in action.

So the ancestors of the Pinacate beetle already had the genetic recipe for making the base stock for the species' chemical weapon. It may be that one prehistoric beetle habitually made more of the chemical than its exoskeleton strictly required, avoided being eaten, then passed that genetic hygiene problem on to its offspring. Evolution would then have refined the delivery method over hundreds of generations, with each improvement --- a better, stinkier recipe; spraying rather than just passive stinking; a headstand early warning system -- providing additional likelihood that its possessors would live long enough to pass those new traits on to their offspring.

Eleodes aren't the only beetles who've adopted this defensive technology: most of the more than 40,000 species in the larger ground beetle family to which Pinacate beetles belong use chemical defenses. The practice reaches its fullest flower in two separate groups of bombardier beetles, which mix their quinone compounds with hydrogen peroxide in a sealed abdominal chamber. A chemical reaction between the two creates acrid caustic steam, which the beetle then blasts at its foes.

Bombardier beetles, incidentally, are favorites of Creationists, who claim that such a fine-tuned defensive system could not have evolved part-way. But hydrogen peroxide is a byproduct of cellular metabolism, and some beetles mix it with quinones without a pressure chamber when threatened, which causes an icky froth to exude from their orifices. The only evolutionary innovation separating bombardier beetles from the rest is a valve under voluntary control at the beetle's hind end, and it's not like evolution hasn't performed that trick before.

The Pinacate beetle might not know it's a walking rebuttal to the creationists, but that's okay. Besides, evolution is a two-edged sword. A desert rodent called the grasshopper mouse has devised a fiendishly crafty way of circumventing the Pinacate beetle's defenses: the mice sneak up and grab the insect with both forepaws, stick it butt-first into the sand, and eat it from the head end. If that trick catches on, Eleodes is going to have to alter its evolutionary strategy somewhat.

About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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