This is a Silvery Checkerspot caterpillar, photographed at Creekside Reserve in Beavercreek. These caterpillars often feed gregariously, in groups, on wingstem and related plants.
This is the caterpillar of our most common butterfly, the Cabbage White. This species feeds on cruciferous vegetables. This particular caterpillar was photographed in a mixed vegetable/flower garden. It had apparently wandered away from its host plant onto some nearby chrysanthemums.
This is the easily-recognizable Monarch butterfly caterpillar, feeding on its host plant, milkweed. Monarchs ingest toxins from milkweed, making them poisonous to predators such as birds.
Baltimore Checkerspot caterpillar. This butterfly species is rather uncommon and restricted to good-quality wetlands, where its primary host plant, white turtlehead, grows. (Later instars can also use other plants besides white turtlehead.) I found this particular caterpillar at Siebenthaler Fen on May 21, 2016.
This caterpillar will transform itself into an American Lady butterfly. These caterpillars are sometimes darker in color, but this is a particularly beautiful, ruby-red individual. Photographed at Sweet Arrow Reserve, near Bellbrook.
This is a Viceroy caterpillar, feeding on willow. Like many caterpillars, this species is a bird-dropping mimic. (Note that the Red-spotted Purple caterpillar is extremely similar. These two species are difficult to tell apart in the larval stage. There are minor differences in the spines and horns, and it can be helpful to note the host plant.)
A pair of Question Mark butterfly caterpillars. I found them feeding alongside the Great Miami River in Miamisburg. These caterpillars look quite formidable with their bristly spines, but it is actually the "fluffy" caterpillars that you need to watch out for. Most of our really bad stinging caterpillars look fuzzy rather than spiky. (But there are some exceptions to this rule!)