by Joseph DeSisto
Today’s article is about the hemileucines, caterpillars with venom-injecting spines. While most have harmless, if painful, stings, a few have venom powerful enough to kill an adult human. But before things get too dark, let’s take some time to appreciate some harmless little beasties:
These are the newly-hatched caterpillars of the io moth, found in eastern and central North America. As caterpillars, they like to move and feed in groups, which provides some protection against predators. The moth, with a wingspan approaching four inches, looks like this:
Not bad, right? Sadly, for all its showy appearance, io moths don’t have mouthparts and cannot feed as adults. A lucky moth dies when its energy reserves run out after about a week, if it can manage to avoid being snapped up by bats, spiders, and other hunters.
This has its benefits — a moth with no appetite can spend its time and energy mating and laying eggs as much as possible. It also has consequences — the caterpillars, to prepare for the most important week of their lives, have to eat a lot. Within a few weeks of devouring as much greenery as physically possible, an io caterpillar can go from being a half-inch-long worm to a nearly three-inch-long monstrosity, brilliant green with red and white racing stripes:
See the spiky, poisonous-looking tufts growing out of the caterpillar’s back and sides? Io caterpillars are indeed capable, and more than willing, to deliver a painful sting. If you brush up against these spines, the tips will break off and start to inject venom. It’s not nice, but not much worse than an encounter with stinging nettle plants which, incidentally, uses many of the same toxins. The strategy is shared by other hemileucine caterpillars, among which are some of the largest and most conspicuous moths in North America.
Southern Brazil has its own hemileucines, including members of the genus Lonomia. Lonomia moths are large and attractive, if perhaps a bit less flashy than their northern brethren:
Their caterpillars, too, are less conspicuous, which is why they sometimes wind up stinging people. A single sting contains only a miniscule amount of venom — not nearly enough to do any real harm. The problem comes from the fact that hemileucine caterpillars tend to feed in groups. If a person accidentally brushes up against 20 or more Lonomia at the same time, the result can be severe.
Unlike the io caterpillar, Lonomia has venom designed to do more than just irritate the skin. As soon as it enters the bloodstream, anticoagulants work to prevent the blood from clotting, while other proteins punch holes in the victim’s blood vessels. The result is violent internal bleeding. In the worst cases, death is usually the result of internal bleeding in the brain (Pinto et al. 2010).
Over the last few decades, cases in southern Brazil have been on the rise, and scientists at the Butantan Institute in São Paulo have been studying the molecular aspects of Lonomia venom. The most important result is that Brazilian hospitals now have access to an antivenom that quickly halts the venom’s action.
To North Americans a venomous caterpillar, even a deadly one, might seem like an esoteric problem. Yet in parts of southern Brazil, the caterpillar causes more medical emergencies than any snake, spider, or scorpion (Pinto et al. 2010). The Brazilian Ministry of Health estimated that in 2008, for every 100,000 people in the caterpillar’s range, there were eight stings. Meanwhile, incidents have become more common as rain forest has been replaced by fruit tree plantations, which happen to provide ideal habitat for Lonomia, and the stage for deadly encounters.
Pinto A.F.M., M. Bergerm J. Reck, R.M.S. Terra, and J.A. Guimaraes. 2010. Lonomia obliqua venom: In vivo effects and molecular aspects associated with the hemorrhagic syndrome. Toxicon 56(7): 1103-1112.