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Oil Beetles: One of Nature’s Tiny Celebrities

Black oil beetle, Meloe proscarabaeus, quite a purple one.
nedomacki/Getty Images/iStockphoto
Black oil beetle, Meloe proscarabaeus, quite a purple one.

It’s an odd feeling to meet a celebrity and not recognize them until later. There is a sudden embarrassment: “How could I have been so stupid?” Some wiser friend shakes their head sympathetically when they hear the story, but you can tell they’re judging you. You want to go hide under a rock. And picture this: you realize that it happened twice.

Walking along the bicycle path in Polson one April afternoon, I was surprised to see a thumb-sized lump of coal lumbering with great purpose across the pavement. Insect-starved just as much as I was sun-starved from a long winter, I rushed over and squatted down to find a black insect with an iridescent-blue sheen, its head and thorax built like a husky ant, and its abdomen fat yet sleek like a huge, black clove of garlic. Obviously in a rush, the little beast looked up in angry consternation when my boot blocked its path. Its sharply kinked, ruffled black antennae flailed above gnashing jaws.

“Oil beetle,” I heard a British-accented voice in my head, remembering a close friend’s explanation when I had found a similar insect on the roadside in Scotland. The dark little monster was so fervently munching blades of grass that I could hear it from several feet away. “They’re flightless,” she had said. “Oh,” I muttered, eyeing the tiny wing cases that ran hardly a quarter of the way down its massive behind.

Back in Montana, I headed home for some research and had my celebrity-recognition embarrassment, fortunately alone in my apartment. Named for the oily, poisonous droplets that they secrete from their joints when disturbed, oil beetles of the genus Meloe and their family, Meloidae, the blister beetles, have been insect apothecaries for millennia. Cantharidin, the odorless terpenoid chemical responsible for their intense toxicity, causes painful blisters upon contact and can be lethal if ingested. It was used by ancient Egyptians more than 3000 years ago to induce contractions during childbirth and the ancient Greeks prized it variously as an aphrodisiac, healing salve, and poison for assassinations or executions (presumably, at very different doses). For their diverse uses and fascinating ecology, oil beetles were named the 2020 insect of the year by an entomological society in Europe.

More impressive even than these accolades is their life history. Oil beetles are hypermetamorphic, meaning that they go through even more metamorphic stages than most other insects. Beginning life as eggs laid on the ground or near flowers, they hatch into highly mobile, louse-shaped larvae called triungulins that climb onto vegetation to wait for pollinating bees. Some species even exude the female pheromones of their particular host bee, or aggregate in groups to mimic a female bee’s appearance. When a male approaches, they climb onto his body and cling to him until he mates with a female. This is their goal: the triungulins then transfer to the female and hitchhike to her nest, which could be a hive for honeybees or an underground tunnel for some solitary bees. They then drop off and metamorphose into a sedentary, grublike form that feeds off of the pollen that the female has left for her own babies, sometimes eating the eggs and young themselves. From there, they will molt as they grow, pupate, and spend the winter cozily in the bees’ home, before emerging as an adult in the spring.

Then, it’s back out into the sunlight, and the spotlight, to meet their adoring fans. The timing of emergence for different oil beetle species depends on the breeding cycle of their hosts. Here in Montana, one can find the black meloe (Meloe niger) in April and May, and the similar Meloe impressus in late summer into fall. Of course, you never know when you might run into one of nature’s celebrities. Next time you see an insect you don’t know, don’t forget to try some background research; you may have met another tiny A-lister on their way through town.

I’m Charles van Rees from the Nature Guys podcast, here today for Field Notes, brought to you by the Montana Natural History Center, providing natural history education for schools and the public throughout Montana. For information on upcoming events and programs at the Center, please call 327.0405 or visit our website at

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