Spiders are not the best collectors for purposes of identification. But these punctuation butterflies do have this very weird field mark, the punctuation mark.
After I pulled the spider-bundle apart, and teased open the wings, there it was, clear as day, a question mark. This is, then, a specimen of Polygonia interrogationis, the Question Mark butterfly. (See Bev of Burning Silo's photo of an Eastern Comma, showing the comma, here.)
I'm sure that this is not the butterfly I'd photographed earlier. Its hindwing, top side, was not nearly as dark, and its colours not so rich. And the webbing it was wrapped in was pretty grubby, as if it had been hanging around for a while. So all that this find establishes is that there are Question Mark butterflies around here.
But John of A DC Birding Blog left a comment on the earlier post to say that the markings on that living butterfly indicated Question Mark, not Eastern Comma. I quote:
According to Kaufman's butterfly guide, Question Marks have a black dash near the tip of the forewing on the top side, but Commas lack this dash. Both Commas and Question Marks have a series of three black dots together in the middle of the forewing. The Question Mark's black dash is just to the outside of this trio. Your butterfly has it, so I would say Question Mark.
Now I must go out and look for an Eastern Comma...