Wow--you're lucky--they are definitely not tent caterpillars. The mostly black one might be a Silvery Checkerspot; they do use sunflowers as a host plant. Now the ones with the red--that looks to me like a Mourning Cloak larva, and I think it's a bit uncommon for them to be on the subflower plant, but there might be reasons like what Larry (Ladobe) mentioned about the American Ladies on the Echinops on another thread. You are so fortunate to have these! Sorry about your sunflowers though.

Same problem! I have a catalpa I planted about 16 years ago and love dearly. It's had increasing infestations of catalpa sphinx caterpillars over the last several years. I'm always inclined to leave nature to rebalance problems, and we do have more parasitic wasps now than there were at first, but the tree is actually starting to show stress now. There were more dead branches this spring than ever before, and what has really alarmed me is that it's had only a handful of blossoms this year. I pulled off as many caterpillars last year as I could, but the tree's very tall now (the link below is to photos of the tree). I am vegetarian and not inclined to offer the caterpillars as bait for fishermen even if I could collect most of them. Just today learned what the pupae look like. Have seen them before but couldn't find any yet today, digging around the base. I guess they've already emerged, since I found one egg mass. All advice sought and appreciated! Serious poison is not an option, but will consider substances of plant origin like neem. Here is a link that might be useful: Catalpa vs. the Sphinx

"I know they're caterpillars. I don't need to have them identified." As kimmq says caterpillars do not commonly live under stones. Also I know of no caterpillars which would have any relationship with compost. Grubs of other insects often do. Of those grubs some are harmless, or even useful, while others are not something you want in the garden. Identification is important for you to receive meaningful and relevant advice. Your question is not as straight forward as you might think.

It was moved to stop a dog (perhaps) or person from eating it? It is fine to move caterpillars so they won't get stepped on, but they are subject to being eaten/parasitized by something no matter where they are placed outdoors. Once injured, freezing and discarding is a practical method. Ermine moth ( family Yponomeutidae) caterpillars are not hairy. https://bugguide.net/node/view/86/bgimage?from=0