The duskywing identification challenge

Duskywings look like moths, but they are in fact butterflies. — Photo by Matt Pelikan

I’ve always had a fondness for groups of organisms that are closely related and similar to each other. There’s a certain elegance in the way nature improvises on the theme of a good idea, with a common ancestor rapidly (by evolutionary standards) diversifying into a cluster of species, resembling each other but each occupying its own niche.

The duskywings are a perfect example. These are butterflies in a particular genus, Erynnis, five species of which inhabit the Vineyard. Drab and only about an inch in wingspan, they don’t really fit the popular notion of a butterfly, and the untrained observer, encountering two different species of duskywings, might not even notice a difference. But butterflies they are, and to my mind they present a fascinating mix of similarities and individuality.

As is the way with butterflies, our duskywings differ in what type of plant their caterpillars feed on. Three of our species feed on oaks of various kinds; a fourth feeds on aspen; and the fifth, on a common wildflower of dry, sandy areas called wild indigo. Oaks, of course, are nearly inescapable on the Island, and it’s easy to find places where wild indigo overlaps with oak. There are even a few spots — the northwestern corner of Correllus State Forest is one — where these plants occur along with aspen, meaning that all five duskywings can occur in the same spot. I’ve found all of our duskywings along the fire lane that heads east from County Road along the northern edge of the State Forest.

As is also the case with related butterflies, the duskywings differ at least a little in the timing of when their adults are on the wing. But right about now, sometime during the second half of May, the flight periods of all five species overlap. So, in theory anyway, you could find all five in the same place at the same time! The most I’ve managed in a day is four species, but trying for a clean sweep is one of those pointless but amusing exercises that makes insect-hunting so much fun.

The similarity of duskywings makes identifying these butterflies a challenge, though it’s a challenge that eases rapidly once you learn a few tricks. Rapid, bouncing flight and a strong territorial tendency are behavioral traits that help one spot duskywings in general. But beyond that, these are basically dark brown insects with mottled patterns that are difficult to describe or remember.

Happily, one unambiguous trait divides our duskywings into two groups: the presence of so-called hyaline spots — little translucent windows in the wing — distinguishes three of our species, while the other two lack these details. And of the three with hyaline spots, two species feature large spots while on the third, the spots are tiny. So these spots are a critical aid for identification. Duskywings often perch, helpfully, with their wings spread wide, making it easy to look for hyaline spots.

Named after Roman poets, Juvenal’s and Horace’s duskywings are the ones with the big hyaline spots. Juvenal’s is invariably the first duskywing to fly each spring, usually by the third week of April, and is among our most common butterflies. Horace’s is rare and, unlike Juvenal’s has flight periods in both spring and summer (so a “big-spotted” duskywing in late July is surely a Horace’s). Otherwise, these two are hard to tell apart: a pair of pale spots on the underside, present on Juvenal’s but absent on Horace’s, is the most helpful field mark.

The “small-spotted” species, wild indigo duskywing, also has spring and summer flights but is fairly easy to identify because its distinctive hyaline markings. Also, this butterfly is rarely found more than a few feet from its host plant, so location offers a useful cue.

Of the “spotless” duskywings, the sleepy duskywing, an oak-feeding species, is by far the more common, usually on the wing in respectable numbers from late April through the end of May. It is a small and dark species, and after years of watching duskywings, I can often pick this species out even in flight by these characteristics.

Even smaller is the final duskywing, dreamy, which has the aspen-feeding caterpillars. Flying in late May and early June, it features extensive grayish frosting on the outer part of its wings. It’s the prettiest duskywing (which isn’t saying much), but sadly it’s also the least common on the Vineyard; I’ve only encountered it here a handful of times and have only managed one decent photograph of it, years ago in my pre-digital, slide-shooting days.

In the grand scheme of things, these are not particularly important insects. They’re neither helpful nor harmful from the human perspective, and their disappearance would likely have little ecological effect. But I value my acquaintance with them: knowing these insects, I feel like I have a secret perspective on the Vineyard landscape. And the season’s first Juvenal’s duskywing, bopping across a path or clearing, is always a welcome sign that spring is here to stay.