Sandy found this caterpillar on one of our apple trees on June 1, 2012. It was fairly good sized (over an inch long).
We tried rearing it, but unfortunately it didn’t mature, which leaves us with yet another of those hard-to-identify green caterpillars.
Since the odds favor it being one of the “Owlet Moths” in the very large family Noctuidae, this looks like a job for my copy of “Owlet Caterpillars of Eastern North America”. After going through practically all 500 pages of caterpillar images, and a lot of close-but-not-quite possibilities, I have decided on Bailey’s Pinion, Lithophane baileyi . BugGuide doesn’t have a picture of the caterpillar yet, but trust me, my pictures match the pictures in the book almost perfectly, right down to the kind of translucent color, the not-quite-stripes made up of tiny white dots, the pale bands at the segment junctions, the exact pattern of larger dots on each segment running down the back, and the green, almost featureless head.
The only thing is, the book doesn’t list apple as one of its foodplants, although the listed plants (alder, buckeye, currant, hickory, birch, jack pine, and poplar) are diverse enough that apple doesn’t seem like much of a stretch.
Here’s the rear, for good measure. I don’t know what that brown object is. Its location suggests that it is a bit of frass, but it doesn’t really look like what one normally sees squeezed out of the rear of a caterpillar. If it is frass, then that could be another diagnostic feature for identifying the caterpillar. And incidentally, that’s often a good way to find well-camouflaged caterpillars: look for plants with a lot of frass underneath.
 I don’t know who Bailey is, although we do know from the name that it is named after Bailey, not discovered by Bailey. I understand that’s one of the taxonomic naming rules: you can name a species you discover after anybody or anything other than yourself.
 I don’t know for sure about the species I have here, but some of the other closely-related Pinions have been observed attacking and devouring other caterpillars. So given that they are obviously pretty omnivorous, there’s a pretty good chance that they aren’t all that choosy about the leaves they eat, either.
 Frass = insect poop. Most of the time, frass from caterpillars is little cylinders with deep grooves in the sides.