Compton Tortoiseshell Butterfly

2022 March 20

Sandy found this one for me on March 14, 2022 while she was out on a lock job[1]. She said it was hiding out behind a door, and had clearly overwintered there. So she caught it, and brought it home for me to photograph.

Between the mottled colors and the irregular margin of the wings, it was pretty good at pretending to be a dead leaf. The antennae were quite long and upright, and the bulb at the end of the antennae were tipped with yellow.

It was perfectly happy to pretend to be a leaf while I got pictures from various directions, but I could not get it to hold its wings open long enough to see what the top of the wings looked like. I worked on it for 20 minutes, but the instant the wings opened it started flying, and as soon as it landed it slapped them shut again. This was the best picture I got of the top of the wings, and it is awful. The only thing I can see from it is that the top of the wings were dark with lines of orange spots.

Of course, by the time I got home, everybody else had gotten a good look at it including a view of the wing tops. Sam went through our guide to the butterflies of Michigan, and found a good match: it is a Compton[2] Tortoiseshell, Nymphalis vaualbum.

This identification is based as much on how we found it as on what it looks like. This is one of the very few species of local butterflies that overwinters as an adult, sheltering in outbuildings, under bark, and in other protected places. Once it warms up, they fly out to feed on juice from rotting things and nectar from early-blooming trees like willows. They then lay eggs on aspens, willows, and birches, with the caterpillars maturing around the end of July into August. The butterflies then spend a few months flying around, presumably mating and finding a good place to spend the winter.

It amazes me that there are any butterflies that can get through the winter like this, but the Compton Tortoiseshell not only survives the winter as an adult, they also range all the way up to central Canada and Alaska. They must be incredibly cold-hardy. I now know of at least two examples of nymphalid butterflies that overwinter as adults: this one, and the Mourning Cloaks, Nymphalis antiopa. These are both in the same genus, so it is possible that they had a common ancestor that overwintered like this.

They are more distantly related to monarch butterflies, which emphatically can’t overwinter up here, and so end up migrating to Mexico for the winter. Still, if their relatives can do it, I see no reason why it might not be possible to either breed or genetically modify monarchs so that they can overwinter here, too. Wouldn’t that be fun?

[1] Sandy is a professional locksmith, and so spends a lot of time dealing with people’s door issues.

[2] I originally had this as “Compton’s Tortoiseshell”, since one of its close relatives is “Milbert’s Tortoiseshell”, and I figured it would take the posessive form of the name, too. But, it looks like everyone actually calls it just “Compton Tortoiseshell”. I don’t know why the one is written as if it belongs to Milbert, while the other is just named after Compton, but there it is.

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