Every once in a while during our decade-plus sojourn in the Upper Midwest, I’d wonder where all the fireflies were. Because it always seemed that I rarely saw them. I don’t know if that’s peculiar to the Twin Cities area, or Minnesota, or the Upper Midwest more generally, but it just seemed like there were very few fireflies (or, as we called them as kids, lightning bugs). I’d assume that I was imagining that phenomenon (or lack of one, I suppose), except that on moving back to Pennsylvania I was struck once again by how we can watch the fireflies dance every summer evening. They’re everywhere and always something of a mystery and...