A hiker in southern Brazil leans a hand against a tree trunk and feels a sharp burn. No snake, no spider. The hand has landed on a cluster of Lonomia caterpillars, the larvae of a giant silkworm moth whose adults are harmless and often have no working mouthparts. The caterpillars look like lichen or a knot in the bark, and they rest together in dense groups low on the trunk. Their hollow, branching spines pierce skin, break off, and deliver venom from many insects at once.

The result is a syndrome called lonomiasis. The venom forces the body to burn through its clotting proteins until none are left, which can mean bleeding from the gums, from old wounds, and into the brain, along with ruptured red blood cells and acute kidney failure. This episode explains why a venom so toxic still kills only about 1.7 percent of victims, why encounters are rising as orchards replace forest, and how researchers hope to turn one of its enzymes into a new drug against dangerous clots.

• The genus has 14 recognized species spread across South America and Central America, with Lonomia electra reaching as far north as Mexico.
• The lethal dose for an 18 to 20 gram mouse is listed at just 0.19 milligrams given intravenously.
• Fatalities stay low because each spine carries a tiny amount of venom, so the danger depends on how many caterpillars are pressed against the skin.
• The caterpillars have adapted to plum, peach, and avocado trees, which puts farm workers and children in contact with them more often.
• Lonomia obliqua is the subject of hundreds of medical studies. The aim is to copy its clot-dissolving enzyme with recombinant DNA in bacteria or yeast, leaving the kidney-damaging toxins behind.