Common Name: Blister Beetle – As a defense against predation, the beetle secretes a chemical called cantharidin; human epidermal contact results in blistering.
Scientific Name: Order Coleoptera – The Greek koleo meaning covering and pteros meaning wings; beetles have sheathed wings. Suborder Polyphaga – The Greek poly meaning many and phagein meaning ‘to eat’ indicating that these beetles subsist on a wide variety of foods. Family Meloidae named for the type genus Meloe.
Potpourri: Beetles are bountiful. Coleoptera is the largest order of insects and comprises about 40 percent of all arthropods and 25 percent of all described animals; over 250,000 beetle species have been identified. They vary in size from a fraction of a millimeter to 20 centimeters in length and from tenebrous brown to iridescent and striped in color and hue. Having evolved in the Permian Period 280 million years ago, fossil evidence is testimony to the resilience of macroscopic beetle physiology as it has changed little over the millennia. The folded wings under a hardened cover called an elytron (the Greek word for sheath) that define the beetle order provide necessary and sufficient protection to allow for scavenging under abrasive habitats like rocks and tree bark while retaining the mobility afforded by flight. The observation that different species had nearly identical outward appearance was troubling to taxonomists, as on the microscopic scale the diversity manifest in myriad beetle species required resolution. The obscure French entomologist René Jeannel (1879 – 1965) spent most of his life studying the speciation of nearly identical cave beetles, discovering that that the differences were quite literally seminal; they all had different aedeagi. Derived from the Greek aidoia meaning genitals, the aedeagus is the male reproductive organ of insects. Jeannel discovered that genitalia, perhaps not surprisingly, play a key role in speciation and evolution, the subject of the recent treatise Nature’s Nether Regions by Menno Schilthuizen that goes beyond the ‘lock and key’ thesis to sexual selection by females. Beetles have evolved through speciation to inhabit almost every imaginable niche including coprophagous dung beetles, carnivore ladybugs and necrophagous carrion beetles; blister beetles instantiate beetle adaptation in metamorphic complexity and in the chemical complexities of body fluids.
Coleopterans, like their cousins the butterflies and moths of Lepidoptera and the flies of Diptera, undergo complete metamorphosis known in biological lexicon as holometabolism – egg, larva, pupa, and adult. The grubs found digging in the garden are the larvae of beetles as the maggots of putrefaction are the larvae of flies. The first blister beetle larvae, according to Stephen Marshall of the University of Guelph in Insects, Their Natural History and Diversity, were found ensconced in the tiny hairs that cover bees; they were scientifically designated Pediculus apis or bee lice to reflect this association. Further study revealed that the bee louse was really the first instar (larvae molt multiple times in some species; each molt constitutes a new instar) of a blister beetle called a planidium. Like the planets which share the same Greek root planis, planadia are wanderers with high mobility that evolved to pursue and parasitize other animals. Blister beetle larvae have three claws on each of foot, a specialized type of planidium called a triungulin (ungula is Latin for claw). Bee-riding blister beetle triungula are among nature’s more notable subversives, lying in wait on flower petals for transport to their trophic beehive haven. On arrival the bee eggs are consumed as an appetizer for the more substantial pollen and nectar, the triungulin now fattened and energized to molt into a second instar in the form of a fat, immobile larva; the dual larval life-cycle is called hypermetamorphosis. As there are some 300 different blister beetle species in the family Meloidae in North America, the bee egg-eaters are but one of many variants; one species eats grasshopper eggs to check perhaps the most voracious of agricultural pests. While differentiated in diet, blister beetles are alike in chemistry, they all produce cantharidin.
In the struggle for survival that is almost wholly devoted to finding something to eat and not being eaten, plants and animals evolve according to random mutations that give them better odds of reproducing. Blister beetle males produce an odorless, yellow terpenoid organic chemical called cantharidin that is an irritant to many animals and deadly to some. The primary function of cantharidin oil produced by blister beetles is to protect the eggs that result from successful mating; it is transferred from the male to the female during copulation. A male’s potential for progeny is in direct proportion to the quality and quantity of cantharidin, a factor that certainly contributes to its ubiquity. The efficacy of cantharidin is manifest in numbers; there are about 2,000 different species of blister beetles that have evolved from the original mutant (assuming that they are monophyletic in having a common ancestor which only DNA analysis can resolve). Among the more common blister beetles are the 300 species from the family Meloidae, which are also frequently called oil beetles due to the oil-like exudate that emerges from the jointed legs (arthropods are literally joint-legged). They are notable also for their un-beetle-like appearance; they have no hind wings and the elytra cum wing covers do not meet in the middle of the back. In the complicated interactions of nature, it should come as no surprise that there are several species of midges that have adapted to attack blister beetle leg joints to consume the chemical repellant, presumably to use it for their own defensive advantage; a variety of other insects eat dead blister beetles presumably for the same reason. The effect of cantharidin on mammals, notably horses and humans, is an unintended consequence of its primary purpose in propagation.
Cantharidin can kill. Horses are especially susceptible due not only to the virulence of the chemical but also to the vector of its dietary introduction; other grazing ovine and bovine animals are also affected to a lesser extent. Several species of blister beetles swarm in hay fields, the dense concentrations of pollen and nectar provide an ideal trysting and mating habitat. Those caught in flagrante delicto are gathered with the hay crop when harvested; the cantharidin from their carcasses is not dissipated by the heating and desiccating of hay processing and is therefore insinuated into baled horse fodder. Consumption of only about two beetles, or a dose of about one milligram (mg) cantharidin per kilogram (kg), is enough to kill a horse after a series of symptoms including congested and ulcerated oral mucous membranes, abnormal heart palpitations, fever and muscle stiffness that results in a characteristic goose-step gait. There is no antidote for cantharidin poisoning and the only treatment is palliative care. Humans are equally susceptible to the toxin at a horse consistent dose rate of about 1mg/kg body mass; beetle ingestion by children is one notable introduction vector. Given that cantharidin is a blistering agent, burning of the oral cavity and the esophagus is almost immediate followed by a total loss of mucous in the gastrointestinal tract; renal failure is the normative cause of expiration. The magisterial Britannica lists cantharidin under animal poisons with the caveat “a very dangerous substance to use; ingestion can cause sever gastroenteritis, kidney damage, blood in the urine, priapism, profound collapse and death.” So why would anyone want to use it? Priapism offers a clue. Priapus, the son of Aphrodite, was the Greek god of procreative power.
There is another more insidious aspect of blister beetles and cantharidin that may result in intentional human ingestion. A blister beetle is the source of the near universal cultural aphorism that Spanish fly is the quintessential aphrodisiac (a hackneyed bon mot is that it is called American fly in Spain). The Spanish fly trope is of ancient provenance, and probably precedes recorded history; it is the common name of the blister beetle Lytta vesicatoria native to southern Europe. The association between cantharidin and Spanish fly is so implicit that the word cantharides, derived from the Greek kantharis meaning ‘blister beetle,’ is defined in Webster’s Dictionary as “A dangerous, sometimes fatal preparation of powdered, dried Spanish flies.” While cantharidin has been and is still used as a legitimate medicinal for a variety of conditions, the crossover to aphrodisiac is obscured by the mists of time and by the sub-rosa nature of its purported use manipulation (now known as the Cosby effect). According to a peer reviewed article in a 2001 article in JAMA Dermatology entitled “Cantharidin Revisited,” the connection is one of physiology “based on the observation of pelvic congestion in women and priapism in men;” entomological Viagra. The most notorious, if likely apocryphal use of Spanish fly was by that most salacious of sodomites the Marquis de Sade, who supposedly used it in his legendary sexual escapades. There are, however, several well documented more recent occasions of attempted sexual coercion using the famed Iberian concoction. One case in England resulted in the deaths of two women given coconut ice laced with cantharidin by an unintentionally sadistic and woefully uninformed suitor. In 1996, four undergraduates from Temple University were hospitalized after drinking cantharidin Kool-Aid with the admitted intention of using it on their girlfriends. There was no indication that they had recently read the Tom Wolfe classic. An article appeared in the Clinical Kidney Journal in 2013 about a soldier who accepted the challenge of eating a blister beetle, presumably to test the sexuality hypothesis. He was admitted to an emergency room six hours later with severe abdominal pain and renal dysfunction; he survived though possibly an eventual candidate for the Darwin Award. There is a lesson there somewhere, and it isn’t “don’t ask, don’t tell.”
In spite of its iatrogenic effect if used improperly, cantharidin is potent medicine with historical provenance in Asian, European and aboriginal cultures. Dried Chinese blister beetle has been in use in the Far East as a folk medicine for several millennia both topically for various skin disorders and internally to induce abortion and as an anticancer agent. The noted Greek herbalist Pedanius Dioscorides was a surgeon in the Roman Army in the first century CE and traveled extensively in this assignment. Based on his professional field study experiences which encompassed most of the known world, he published the first western Pharmacopoeia in about 70 CE. Now known by its Latin name De Materia Medica (on medical material) which included cantharidin as a drug, it was the desk reference of choice for practicing physicians until the Renaissance. The Bantu speaking Tswanas of South Africa used ground blister beetles for a substance called seletsa that was used as both an aphrodisiac and an abortifacient; its use reportedly resulted in fatality with some frequency. What one should conclude from this is that cantharidin is a powerful chemical that should probably not be ingested except perhaps as a literal last resort. What it is good for is warts and molluscum, a similar skin viral infection. Cantharidin is absorbed by the epidermal cells to form blisters (remember we are talking about blister beetles here) that form within 24 hours and result in complete wart-less healing in a week. Cantharidin was used for wart remediation in the United States in the 1950’s but in 1962 the FDA amended the Food, Drug and Cosmetic Act to require drug manufacturers to submit efficacy data for their products; since none was received for cantharidin, it was taken off the market. In 2015, the Pharmacy Compounding Advisory Committee of the FDA voted to restore cantharidin as an accepted compounding drug, warts and all. No mention of Spanish fly.