I had a breakthrough of sorts this summer: I learned to identify skippers. A couple of skippers, at least. Thank God for good field guides, in this case Jeffrey Glassberg’s Butterflies Through Binoculars and Jim Brock and Kenn Kauffman’s Butterflies of North America. There’s a real satisfaction in being able to assign names to things, even things as obscure as the umber skippers in my back yard.
Skippers are those small, mostly brown, hyperactive butterflies you may have noticed dashing around your flower bed and passed off as some kind of day-flying moth. They differ from typical butterflies in the shape of their antennae, which have a kink in the clubbed tip, and they tend to have thicker bodies and shorter wings.
In some skippers, males have specialized scales on their forewings that produce pheromones to attract females. There are a lot of skippers—3,600 worldwide, about a third of all North American butterfly species—and, apart from a few flashy tropical types in Florida and south Texas, they all look pretty much alike. But with patience, it’s possible to sort them out.
Lepidopterists believe skippers branched off from the main stem of butterfly evolution in the Cretaceous period, when the dinosaurs were still going strong. The caterpillars of early skippers probably ate plants in the legume family: lupine, locust, lotus, wisteria. Early on, though, one group, the grass skippers, developed a preference for monocotyledonous plants, mainly grasses and sedges. As the global climate cooled and grasslands displaced the old broadleaf evergreen forests of the northern continents, grass skippers spread and diversified along with the bigger grazers like horses and bison.
Skipper caterpillars are modest-looking creatures, without the horns and bristles of some butterfly and moth larvae. After hatching, they roll up a leaf into a tube and stitch it in place with silk. This becomes home base, from which the caterpillar ventures out to feed. As it grows, it constructs successively larger tube nests; in the last one, it transforms into the pupa from which the adult butterfly will emerge.
The caterpillars have one trait that’s really remarkable, and that inspired a recent article in the journal Ecology Letters entitled “Good housekeeping: Why do shelter-dwelling caterpillars fling their frass?” Frass is the collective term for the fecal pellets of larval insects. Years ago on a summer trip through New England, camping under an oak tree that was being defoliated by gypsy moths, I woke up to the patter of frass on the tent roof, like a gentle rain of Grape-Nuts.
Skipper caterpillars, as first described by an entomologist named F. W. Frohawk in 1892, eject their frass from their leaf-tube nests with impressive velocity and range. There’s a structure on their rear end which was originally interpreted as a kind of catapult, but which has been shown to be a mechanical latch in an ejection system driven by a localized increase in blood pressure.
Martha Weiss, the Georgetown University biologist who wrote the “Good housekeeping” article, studied the silver-spotted skipper, a handsome species that ranges from coast to coast. She found that skipper larvae can propel frass up to 39 times the length of their bodies. The older and larger the caterpillar, the greater the distance. With an average score of 19 body-lengths, 39 would be Olympic quality. Compare that with your world-class human shot-putters.
And why do they do this? Weiss came up with 3 hypotheses: sanitation, crowding, and protection from enemies. If the caterpillars let frass accumulate, they might be vulnerable to fungal diseases. Or the buildup might force them to change shelters more often, wasting vital energy. Or they might be eliminating a cue that would lead predators and parasites to their nests.
In an elegant set of experiments, Weiss shoveled frass back into the nests of some caterpillars while allowing control larvae to practice their usual housecleaning. She found that fungi grew on the frasspiles, but that this did not appear to harm the caterpillars, which reached maturity in similar numbers to the controls. And caterpillars repeatedly crowded out of house and home also did about as well as
those that were left alone.
But the predator trials had a dramatically different result. Weiss found that predatory wasps picked off skipper caterpillars in frass-filled nests more often than in untreated nests. They seemed to hunt by scent, ignoring nests with faux frass in the form of small black glass beads. You do have to respect the olfactory powers of wasps; some species can detect spiders in their subterranean burrows by smell. The Defense Department has funded research into the ability of parasitic wasps to sniff out nerve gas toxins or chemical signals from unexploded bombs and mines.
The one question Weiss was unable to answer was, Why so far and so fast? The skipper larva’s system does seem a tad overengineered.
She speculates that the speed and distance of the fling may be just a byproduct of larval physiology.
Nonetheless, it does seem clear that frass-flinging is an advantageous behavior that must have been subject to natural selection—which, after all, can work on anything an organism does, any physical trait or behavior that varies among individuals. It’s an interesting point to contemplate in an election year when the frass is flying fast and furious.