I found this/these apparently glued, together, to our deck railing, I suspect by way of the cocooning wasp, but once pulled away they separated. When I checked the container on 6/7 I was much surprised to find them again attached in a similar way (the caterpillar I could now see grasping the wasp cocoon with most of its prolegs). That sparked a memory of a report I'd read of some other caterpillar parasitoid hijacking its prey into acting as its 'zombie' bodyguard. And, indeed, when I repeatedly lightly touched the rear portions of the caterpillar, near the wasp cocoon, the caterpillar would repeatedly vigorously twitch back and (perhaps) bite with its mandibles. The action seemed reflexive, too quick and too small-scale to see in fine detail or to capture with still photos. The reaction did not cease in the several and more times I tried it. I suspect this is another example of a parasitoid altering the host/victim behavior. Only sorry I don't know how to use my camera video!
The caterpillar is Amphipyra pyramidoides. Though quite small (~ 12-15 mm; hard to measure well), I suspect this is either a penultimate instar or early final instar caterpillar. The dark wound, I assume, is where the wasp larva exited. I hope to obtain an adult wasp to post.
A bit of online searching turned up a photo of a similar situation involving a plusiine caterpillar.