“Brown and furry
Caterpillar in a hurry,
Take your walk
To the shady leaf, or stalk…”
-Christina Georgina Rossetti, “Sing-Song” (1893)
The autumnal equinox has passed, leaves are starting to turn, and throughout the country woolly bears are running rampant across streets and sidewalks, sensing in their fuzzy little bodies the urge to pack on fat before the cold sets in. They appear blind and brazen in their search for food, conspicuously traveling by day, traversing heavily-trafficked footpaths with reckless abandon, all the while humping along at a scorching 0.045 miles per hour. Polyphagous herbivores, woolly bears eat just about anything grown from the ground with chlorophyll in it. Variety, it’s been said, is the spice of life, and the bears go to great lengths to find it. They are the browsers at a buffet line, and God’s green earth their spread.
Generally, woolly bears seen in Washington are larvae of the Isabella tiger moth (Pyrrartica Isabella), but the name “woolly bear” is often used to describe larvae of any number of Arctiid species, a diverse moth family of which there are more than 11,000 species worldwide. Arctiid larvae have the interesting distinction of being far more widely-known than their adult forms—everybody can identify a woolly bear, but scant few have seen or even heard of the resultant moths. Indeed, the fact that tiger moth larvae are called “woolly bear caterpillars” and not simply “tiger moth larvae” only widens the disconnect. But in a way, it makes a lot of sense: The larvae are diurnal, relatively long-lived, and often brightly colored, whereas the imagoes are nocturnal, ephemeral, and cryptically shaded. The caterpillars are the visible stage of its life cycle, eating, creeping, and bumbling about; the adults work undercover, behind the scenes. Banded orange and black, covered in soft bristles, the itinerant woolly bear is also cute, a quality few moths can be said to possess.
There is a great deal of folklore surrounding the woolly bear, centered on its purported ability to forecast winter weather. A vaguely scientific study conducted from 1948 to 1956 by a Dr. C. H. Curran in New York attempted to prove that wider bands of orange (the caterpillar’s middle segment) signaled a mild oncoming winter, while narrow orange bands corresponded to harsher weather. He collected “as many caterpillars as he could in a day” from Bear Mountain State Park each fall for eight years, collating the data and offering to forecast the winter weather for a reporter friend at The New York Herald Tribune. His predictions bore out, more or less, though he himself acknowledged the small sample size and admitted that the trips to the park “were simply an excuse for having fun.” Curran, his wife, and their group of friends gamboled in the woodlands of southeast New York, fancying themselves “The Original Society of the Friends of the Woolly Bear”. Most scientists discount the theory—sample size being the big sticking point—but one, an entomologist formerly of the University of Massachusetts named Mike Peters, thinks that a link between the bands and winter severity is possible: “There’s evidence that the number of brown hairs has to do with the age of the caterpillar—in other words, how late it got going in the spring,” he said in 1999. “The [band] does say something about a heavy winter or an early spring. The only thing is, it’s telling you about the previous year.” Sort of like looking at the width of tree rings to distinguish good growth years (likely due to lots of rain) from bad, dry ones. But unlike trees, which last for centuries in the right conditions, caterpillars tend to decompose rather quickly—not the most useful long-term metereological metric.
While woolly bears likely aren’t smarter than the average ursine bear—what with their pinhead-sized brains—they do appear to exhibit a behavior more often associated with “higher” species: self-medication. A recent study by researchers at Wesleyan University found that the caterpillars of another Arctiid species (not the Isabella), when infected with wasp larvae, seek out and devour more alkaloid-rich plant matter than non-infected caterpillars, in an apparent attempt to kill the parasites growing within them. Some background: Many Arctiids are parasitized by flies and ichneumon wasps, which sting or otherwise subdue the caterpillars before laying eggs on them. The larvae emerge, burrow into the caterpillar, and essentially eat the host alive from the inside out. (Sound cruel? Darwin, in 1860, thought so as well: “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars…”) The woolly bears, seemingly unsatisfied with this arrangement, dose themselves with pyrrolizidine alkaloids to fight back, but it’s a risky maneuver: alkaloids are toxic even to the caterpillar, and too high a dose often ends up killing the host. What makes this finding all the more remarkable is that, unlike humans and other primates that self-medicate, these woolly bears are doing so without being taught. In other words, it appears to be innate behavior, a built-in adaptation to an uncompromising world.
For the next couple months the woolly bears will continue ambling around, searching for food and, eventually, a place to hunker down and pass the winter. Many Arctiid species can survive extremely low temperatures—literally freezing solid without harm—thanks to substances called cryoprotectants in their blood, which inhibit the growth of cell-rupturing ice crystals. In the spring they’ll thaw out, pupate, and become moths for a fleeting coda, the twilight of their days spent gallivanting in the moonshine.