The caterpillar is about 5 cm long, slim and slightly hairy--distinctive enough that I figured I could find out what it was. (Still amazed that any of these creatures can be identified, but I've been on a roll lately.) Turns out, to my great surprise, this is an Eastern Tent Caterpillar (Malacosoma americanum). Out of its tent, and out of the squirmy mass of small caterpillars that I know so well, this is quite a handsome beast. There was talk of a bad year for these caterpillars (that can devastate fruit trees) this year, but in this area they were quite modest. A tent here and there, no trees stripped. The tents are all empty now, but I didn't know that the caterpillars travelled on, growing and eating on their own before cocooning. From what I can glean from various sources, we know this creature by its caterpillar name, it turns into a tent caterpillar moth, one of the Lasiocampids.
The day after I took these pictures I went back to look for the caterpillar, and it was still on the black knot--lending weight to my earlier impression that it was eating it. There was evidence on the bit of knot of nibbling, but not very compelling--the surface is rough and broken looking anyway. Strange....but not particularly a benefit to the tree--once the black knot turns black it's done its work, so eating it won't reduce the infection.
Last weekend I noticed another tenter in the yard--more casual, draping webbing over and around the ends of branches, exclusively infesting Wahoo (Euonymus atropurpureus), of which there are three, about 50 metres apart. This is a native species of euonymus that produces a very pretty fruit in the fall, pink capsules around scarlet seeds. Turns out that it's not a native caterpillar.
The euonymus caterpillar is a European that feeds on the European euonymus, came with that species, and has recently branched out to native euonymus in North America, much like the case of the European sawfly. So clever, so adaptable. The mature form is an ermine moth, called the European spindle ermine moth, after another, closely related species of tree, the spindle tree, it likes to eat. The moth is medium-sized, and white with black spots on its wings.