In the middle of September, 2023, I was hiking along a meadow trail bordering a mixed conifer woodland at about 10,400′ in the San Juan Mountains of Southwestern Colorado. I saw a strange-looking insect that was slowly climbing up the grass next to the trail.
It was an unlikely looking creature, all glossy blacks and greens. It was perhaps an three-quarters of an inch long. It had a long and bulbous, almost bloated, abdomen which protruded well past its wing covers. The way its abdomen protruded past its wing covers gave me the impression of a person trying to squeeze themselves into a ludicrously over-small vest.
This was a member of the family Meloidae – the Blister Beetles. The Meloidae are also called ‘Blister’ Beetles because they contain a have defensive compound, cantharidin, that can cause skin burns and blisters on contact.
There are a large number of Blister Beetles species world-wide, and most of them don’t have this strange, too-large-for-their-clothes look. This particular Blister Beetle is in the genus Meloe, which have a common name of ‘Oil Beetles’.
I’m tentatively identifying this Meloe barabarus, based on photos from Beetles of Western North America, but I’m not certain of that species-level-ID.
Each of the beetle’s antennae had a strange ‘crook’ in them, indicating that this is a male Oil beetle.
These antennal crooks are used like pliers for grasping the female’s antennae during the Oil Beetle’s courtship and mating.
I put my hand under the beetle and tapped its grass stem. The beetle fell into my gloved hand and played dead. Sometimes Blister Beetles will exude droplets haemolymph from their leg joints if disturbed – this is called ‘reflex bleeding’.
Blister Beetle haemolymph is full of a distasteful, caustic chemical – cantharadin. So I was very pleased that this beetle had resisted the urge to reflex bleed.
From what I’ve read, only the males, such as this one, manufacture cantharadin. As part of the Oil Beetle’s mating ritual, the male gifts the female with a massive dose of the cantharadin that he has synthesized. The female sequesters the cantharadin, and uses it to coat the eggs she lays.
These eggs hatch into mobile, legged larva called ‘triungulins’. They don’t look like typical, stumpy grub-like beetle larvae. These have long legs, and are active. The triungulins are so non-grub-like that Linnaeus initially classified triungulins as lice rather than as beetles.
In Oil Beetles, these triungulins climb up onto plants and wait for a bee, usually one of the Digger Bees, to come near. Then they grab on to the bee, and ride it. If the bee is a male, the triungulin will hang on to the male until he mates with a female bee, then switch from the male to the female.
At least one species of Oil Beetle in the Southwestern US, Meloe franciscus, has triungulin larvae which don’t just climb onto a flower and passively wait for a Bee to visit. Instead, this Beetle’s triungilins emerge from their eggs together and move through the landscape as single unit, a clumped group, The traveling clump of larvae will even form living bridges between small obstacles.
Eventually, this cluster climbs to an elevated spot, such as a grass stem, and forms itself into a tight ball of squirming triungulins. The cluster will stay in this location for days. even weeks, and a time, emitting a pheromone which attracts male Digger Bees.
Hafernik and Saul-Gershenz (2000) believe that the triungulins cluster in this manner and emit the pheromone to better imitate a female Digger Bee. The male Digger Bees are trying to mate with what they think is a female Digger Bee, but is really a squirming mass of brood parasites. As the male attempts to mate, triungulins crawl onto him.
The male bee eventually gives up, moving on to try to mate with an actual female bee. But he’s bringing his past relationships with him in the form of a squirming load of passenger triungulin larva attached to his underside.
If the male does find and mate with a female, she’ll acquire a dose of triungulins from the male.
I’m not sure whether the particular species of Oil Beetle in these photographs has triungulins which aggregate and trick male bees, or whether its triungulins singly ‘hunt’ the female bees. I wouldn’t be surprised either way.
Eventually the female bee will return to her nest cell, inadvertently bringing one or more triungulins with her.
Once inside the bee’s nest chamber, the Blister Beetle triungulin, feasts on the provisions that the bee has stored, and kills and eats the bee’s offspring.
So…the earliest instar, the triungulin, is specialized for an active, hunting or seeking life – finding and attaching themselves to a host insect. Now that they are ensconced in the Bee’s nest, surrounded by the provisions the Bee had stored for her (now ex-) larva, the triungulin doesn’t need to be active anymore.
It molts into a second, less active larval stage, one that looks and behaves quite differently, more like a traditional beetle grub.
It loses its long ‘travelling legs’, and its eyes. Its body form changes to one more typical for beetle larvae, a grub-like body that is optimized for growth and feeding, rather than clambering around the landscape.
Many insects have a dramatic change in form and function, a metamorphosis, between their larval and adult life stages. This change happens during their pupal stage. But Blister Beetle larva have an additional “big change” step – the shift between the lithe, mobile triungulin and their ‘grub-like’ form.
This change in larval form and function outside of pupation is called ‘hypermetamorphosis’, and it’s unusual in insects. It seems to be seen primarily in parasites and parasitoids.
Interestingly, I’ve got a post about another beetle this site that also exhibits hypermetamorphosis. It’s not a Blister Beetle – it’s a Rhipiphorus Beetle. And thie Rhipiphorus Beetle, too, has parasitic larva…also on Solitary Bees.
While this was the first Oil Beetle I’ve seen, I have taken photos of other types of Blister Beetles over the years. I’ve not made posts about them, though. Partly that’s because I’m missing photos of so much of their life cycles – the larval stage, as just one example.
But a bigger issue I’ve had is that there are just so many cool things about them.
How on Earth do you stop researching and start writing? And once you’ve actually started writing, how do you determine what to leave out? How could you not mention this aspect? And what about the strange ramifications of that observation? And you simply must mention this aspect of their lives and ecology!
I sincerely doubt that I’ve done a good job here of explaining, or even summarizing, what’s so very interesting about these creatures. But I hope this is a good start. If you’re interested, I heartily recommend that you read some of the articles listed in the “Sources” section below.
The great American poet Tom Waits once wrote “Every thing you can think of is true.” Blister Beetles and the way they fit into the world…yeah, I think they might be an example of that.
Sources:
Hafernik, John, and LeslieSaul-Gershenz. 2000. Beetle larvae cooperate to mimic bees. Nature 405, 35-36 (2000).
Eisner, Thomas and Edward O. Wilson. 2003. For Love of Insects. Belknap Press, First Edition (November 30, 2003). ISBN 0674011813.
Eisner, Thomas, Maria Eisner, Melody Siegler. 2007. Secret Weapons: Defenses of Insects, Spiders, Scorpions and Other Many-Legged Creatures. Belknap Press: An Imprint of Harvard University Press (April 30, 2007). 384 pages. ISBN 0674024036.
Evans, Arthur V. 2021. Beetles of Western North America. Princeton University Press (September 28, 2021). ISBN 0691164282.
Marshall, Stephen A. 2018. Beetles: The Natural History and Diversity of Coleoptera. Firefly Books; Illustrated Edition (September 1, 2018). ISBN 0228100690.
Wikipedia’s entry on Hypermetamorphosis. Most helpful, as usual.
