It’s getting to be very late in the year for moths. Winter approaches, and our warm days now are lucky to make double-digits Celsius (>50°F). Hardly anything comes to the porch lights in the evening now, as by nightfall the temperature has dropped too much for anything to be flying. However, there are still some cold-hardy species out and about.
I discovered this one resting on a leaf in the forest when I was out with Raven a few days ago. If it had been on the trunk of a tree, or a fallen log, or even on a dead leaf on the ground, I likely would have walked right by, but it stood out on the green leaf. It’s a Bruce Spanworm (Operophtera bruceata), one of the latest moths to be on the wing. The first ones appear here in late October, and they fly on warm days (and on warmer evenings) through November. They can handle the cooler daytime temperatures because they’ve got a very high surface:volume ratio – that is, they’re very narrow, with lots of surface area, so they absorb the sun’s rays and ambient temperature more quickly than a thick-bodied moth would. (There are thick-bodied moths on the wing now, too; they’re invariably fuzzy, which helps to insulate them.)
They’re very similar to the Autumnal Moth (Epirrita autumnata), which is also a fall flier (as the name suggests) but not quite as late. They first appear in late September, and their flight window barely overlaps with Bruce Spanworm. However, you can easily tell the two apart by the dots along the outer edge of the hindwing – in the Autumnal they’re paired at the end of each vein, while in the Bruce there’s just a single dot. The Bruce is also slightly smaller. The Autumnal below is one I photographed last year.
The day after I saw and photographed this Bruce, which was the first one I’d seen this season, I spotted another fluttering low to the ground in our front yard. By the time I had retrieved my camera, it was gone. It might well be the last moth I see this year, if the long-term forecast is to be believed.