When you think of venomous animals, caterpillars probably aren’t the first thing that comes to mind. Snakes, of course. Scorpions and spiders, too. But caterpillars?
Yes, indeed. The world turns out to be home to hundreds — perhaps thousands — of species of venomous caterpillars, and at least a few of them pack a punch toxic enough to kill or permanently injure a person. That alone is reason for scientists to study them. But caterpillars also contain a potential windfall of medically useful compounds within their toxic secretions.
“Will we get to the stage where we’ll be taking things from their venoms that are useful? Definitely,” says Andrew Walker, an evolutionary biologist and biochemist at the University of Queensland, Australia. “But there’s a lot of foundational work to do first.”
Caterpillars are the larval stages of the insect order Lepidoptera, the butterflies and moths. It’s just one of many animal groups with little-known venomous members. (Venoms are toxins that are deliberately injected into another animal, while poisons sit passively in an organism’s body, waiting to sicken a potential predator.) By biologists’ best estimate, venoms have evolved at least 100 times across the animal kingdom.
Many venoms are complex, some containing more than 100 different compounds. And they’re also strikingly diverse. “No two species have the same venom arsenal,” says Mandë Holford, a venom scientist at Hunter College and the American Museum of Natural History in New York City. “That’s why it’s important to study as many species as we can find.”
Indeed, studying venoms could be a better way to find new drug candidates than starting from scratch, because they contain molecules fine-tuned over eons to target specific biological processes in the victim. “They’ve evolved over millions of years, they’ve been tested in nature, and we know they work,” says Holford. “When we try to devise them ourselves in the lab, the success rate is a lot less.”
Most groups of venomous organisms, however, are barely on scientists’ radar. “We have a huge wealth of knowledge about snake venoms and scorpion venoms and spider venoms,” says Nicholas Casewell, a venom biologist at the Liverpool School of Tropical Medicine in the UK. “But there are lots of groups of venomous animals out there that are largely unstudied.”
Venomous caterpillars aren’t confined to the tropics. The spongy moth (Lymantria dispar) shown here, is native to Europe, but in North America it is an invasive pest that can defoliate forests. Contact with its venomous bristles can cause an itchy, painful skin rash.
CREDIT: ISTOCK.COM / MARCIN WOJCIECHOWSKI
Caterpillars, in particular, deserve more attention, says Walker, who wrote about venomous Lepidoptera in the 2025 Annual Review of Entomology. Though only about 2 percent of caterpillar species are venomous, by Walker’s estimate, they’re widely scattered across the lepidopteran evolutionary tree. That pattern means venom likely evolved independently several times within the group, potentially yielding an unusual diversity of chemicals. The deadly caterpillars — within the South American genus called Lonomia — have a snakelike venom that interferes with blood clotting. Others have venoms that cause chronic, lifelong inflammatory problems, and a couple cause miscarriages in horses.
Those nasty few are enough to make venomous caterpillars a significant public health issue, in at least some parts of the world, says Walker. “They’re not killing many people regularly like scorpions and snakes do, but compared to spiders there’s not much difference in the impact of the health hazard.” The concern has led some researchers to work on understanding the biological effects of the potentially lethal Lonomia venom, and to develop antivenom to treat affected people.
Though a few other caterpillar venoms have been studied at least a little, most remain almost entirely unstudied, says Walker — and medicine may be missing out. For example, he notes, most lepidopteran venoms cause pain, sometimes intense enough to require opioid painkillers. That’s not surprising, since pain is an excellent way to deter predators — but it also allows researchers to use the venom as a probe to identify pain pathways in the body and pain receptors in lab animals and, potentially, in people. That, in turn, could lead to new drugs.
Research on caterpillar venoms is still sparse enough that no new drugs have resulted yet, but venoms of other organisms have yielded some important therapies. There are blood-pressure and anticlotting medications that were inspired by snake venoms, for example, and a forerunner to the new blockbuster drug semaglutide — better known by brand names such as Ozempic and Wegovy — was based on a molecule extracted from a venomous lizard, the Gila monster.
Thanks to advances in molecular biology and bioinformatics, venoms of all animals, including caterpillars, are increasingly easy to investigate — and that should mean big steps forward soon, says Casewell. “It’s like a treasure trove that’s still sitting out there for us to understand.”
