I found this larva among the aphids (identified as Aphis craccivora by Natalie Hernandez) on scarlet runner beans in photos 1461878 and 1461881:

Photo#1461878

It reached around with what I assume was its head quite a bit, but it didn't leave that position on the plant for two days (at least not while I was watching). On the second day that I visited these aphids, it had stopped moving around, and I thought it might be preparing to pupate. On the third day, it had either moved a few millimeters away, or it had left (or been removed) and a second (active) larva appeared a few millimeters away.

I found one aphid mummy with an exit hole and others that appeared to have been parasitized by wasps, so I was guessing that this was the larva of a parasitoid, but from poking around on the web, it looks like the parasitoids emerge as adults. Do they ever leave their hosts as larvae?