The Beetle That Killed a Racehorse

8 min readAug 13, 2021

Are blister beetles the world’s most bonkers insect? You be the judge.

Phonon.b, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

If you have ever said to yourself, “I wonder if there’s a such a thing as a beetle that eats bees, kills horses, and causes destructive, painful erections,” then you’re in the right place. We’re talking blister beetles here, and there is much to be said.

Blister Beetle Basics

There are almost 2,000 different species of blister beetle, all in the family Meloidae. Beetles, like butterflies and moths, undergo complete metamorphosis, which is to say they have four more-or-less discrete stages of development: egg, larva, pupa, and adult. While butterflies and moths are the ones we typically think of as undergoing this process, lots of other insects do it too, including bees, wasps, and flies.

The blister beetle Lagorina sericea, looking fabulous. Image via Wikimedia Commons.

Complete metamorphosis is mind-blowing in its own right, but the blister beetle has come up with its own sick little spin. It’s a bit complicated, so bear with me as I break it down, step by step:

Step 1: The beetle begins life as an egg, laid on a plant or under a rock by the mama beetle. So far so good.

Step 2: The egg hatches, and what comes out is a spiny, rapacious little nightmare called a “triungulin.” The triungulin is a tiny, ant-like thing that scurries around on long legs, in the words of Harvard entomologist Piotr Naskrecki, like a “shrew on amphetamines.”

Step 3: The triungulins run around on flowering plants looking for a bee to victimize. Some species emit pheromones that mimic the mating call of solitary bees, while some just ramble around hoping to get lucky. Either way, when they get the chance, the little beasts jump on a bee, sink their claws into its back, and hitch a ride back to the bee’s nest.

Step 4: Once the hitch-hiking triungulins arrive at the nest, things get weird. Each triungulin drops off the bee, crawls into one of the cells of the honeycomb and sheds its skin (technically called “molting”). When this happens, the skin on the back of the leggy, spiny triungulin splits open and out comes a smooth, legless grub-maggot. No more triungulin, ever again. In its place is now a featureless white blob with a mouth at one end and a butt at the other.

Step 5: The grub-maggot starts eating. It eats all the food the worker bees bring for the little baby bee, and when it’s done with that, it eats the little baby bee too. When it’s done with that, it starts eating the walls of the honeycomb itself. It basically eats everything it can get its maggoty little mouth on. Since it’s getting fatter and fatter, it needs to shed its skin several times, always staying true to form as a legless grub-maggot.

That is, unless it gets really hot or really cold out. If it does, then this happens:

Step 5A: If the weather turns unusually hot or cold, the grub-maggot sheds its skin and stops being a squishy pale blob. The insect — and remember, this is still technically a beetle we’re talking about — now turns into a hard brown “pseudopupa.” It has no legs so it can’t move, and no mouth so it can’t eat (which is just as well, since it also has no butt so it can’t poop). This optional, environmentally-triggered pseudopupal stage is vanishingly rare among insects. Added to the triungulin stage, it makes the development of the blister beetle among the most complex of any organism on the planet.

Step 5B: Inside the hard shell of the pseudopupa the maggot breaks down into goo, much like a butterfly or moth inside their pupal shell. It also basically stops breathing. It can stay like this for up to a year, or whenever conditions improve, at which point…

Step 5C. The muscles and other structures magically reform inside the hard shell. It shakes and rolls and splits open, and like Derek St. Hubbins bursting triumphantly from his space pod, the insect re-emerges. It is now… a legless grub-maggot again!

Remember, all of this only happens if it gets really cold or hot. It might not happen at all.

Step 6: Whether or not conditions lead it to momentarily become a pseudopupa, the grub-maggot resumes eating until it’s about the size of an adult bee. Then it forms a pupa — a real one — and there it finally metamorphoses into the adult beetle form.

Step 7: The pupa breaks open and the adult beetle emerges. It heads out of the bees’ nest (why the bees don’t all crowd around and sting it to death at this point is a mystery to science, but it may have to do with protective pheromones that make the blister beetle smell like bee spirit). The grown-up blister beetle flies off to find another beetle to mate with. The fertilized female lays eggs and the process repeats.

Confused? Me too. Here’s a graphic that probably won’t help:

https://www.sciencedirect.com/science/article/abs/pii/S0022191012000455

This process is called “hypermetamorphosis,” and it is unique to this group. If hypermetamorphosis were the only thing that sets the blister beetle apart from everyone else, it would be remarkable.

But it’s not. There’s more. Much more.

Chemical Weapons

The name “blister beetle” is richly earned. The insect secretes a substance called “cantharidin,” a milky poison that oozes out of the bug’s knees. Because they’re so toxic, blister beetles are usually brightly colored, which is called “aposomatic coloring.” This term refers to colors and designs meant to warn away birds and other predators who might be tempted to try to take a bite. Lots of animals, especially insects, use aposomatic coloring. Blister beetles are also soft-bodied, because you don’t need a hard protective shell if no one’s trying to eat you.

When blister beetles aren’t busy being terrible, they’re busy being gorgeous. Image via Wikimedia Commons.

Cantharidin is something else. It only takes a tiny bit on your skin to cause a painful and long-lasting reaction, and a severe exposure can land you in the hospital. I could tell you what happens when you mess with a blister beetle, but it’s easier to just show you:

What you get from messing with a blister beetle. Image via https://entnemdept.ufl.edu/creatures/urban/medical/blister_beetles.htm

These nasty blisters are painful, but won’t typically send you to the emergency room. You can achieve that by taking a pill laced with cantharidin in an ill-advised attempt to get an erection. We’re talking here about the legendary “Spanish Fly,” a kind of primitive Viagra that you used to be able to send away for in the back pages of awesome magazines like True Detective and Sir! The sketchy mail-order version was 100% fake (and even admitted that fact by using the adjective “spurious” as if it meant “super-effective”), but blister-beetle Viagra actually does have a basis in fact. There is an actual “Spanish Fly” — it’s the blister beetle species Lytta vesicatoria.

The real Spanish Fly, Lytta vesicatoria. Image via https://commons.wikimedia.org/w/index.php?curid=2105311

Ground-up L. vesicatoria beetles have been used as an aphrodisiac for a very long time in much of Europe. The reason it’s used as an aphrodisiac is this: When men ingest cantharidin, it irritates their insides, especially the bladder and urethra, which results in, you guessed it, swelling. So sure, Spanish fly actually can give you a boner, but it’s not the good kind — it’s a painful side effect of a system-wide, potentially fatal poisoning.

I routinely instruct my history students not to quote Wikipedia, but this bit about the biochemical wrecking ball that is blister beetle juice needs some space:

“Cantharidin is dangerously toxic, inhibiting the enzyme phosphatase 2A. It causes irritation, blistering, bleeding and discomfort. These effects can escalate to the erosion and bleeding of mucosa in each system, sometimes followed by severe gastro-intestinal bleeding and acute tubular necrosis and glomerular destruction, resulting in gastro-intestinal and renal dysfunction, organ failure, and death.”

I don’t know what “acute tubular necrosis and glomerular destruction” is, but I do know that no boner is worth risking it.

Blister Beetle, Meet My Friend Flicka

Horses have had their issues with this insect. It turns out that there are several species of Meloidae in the western United States that feed not on bees but on grasshoppers. The process is basically the same, with fast-moving, hungry triungulins chasing down the grasshoppers, riding them to where they lay their eggs, and eating the eggs and immature grasshoppers. The adults emerge from the ground to feed on forage plants like grasses and alfalfa.

The trouble begins when the forage is harvested to feed livestock, specifically horses. The beetles get swept up and crushed in with the horse fodder, and if you just read the above bit from Wikipedia, you know that’s going to be problematic. Sure enough, horses that eat the beetle-filled fodder are often poisoned. In some cases, they die. Blister beetles have killed prized race horses.

Proof that you can’t outrun your problems. Image via Pixabay.

Seems We Just Get Started…

There’s more — people dying after being secretly dosed with Spanish fly at a bar, for example — but at some point when you’re dealing with blister beetles you just have to say “enough!” Please stay safe out there, and remember — crushed up toxic beetles are no substitute for actual medicine!