This is a swarm of recently-emerged caterpillars that soon will become Milkweed Tiger Moths, also known as Milkweed Tussock Moths. They’re devouring their namesake plant for one of the reasons that Monarch Butterfly caterpillars consume it. The toxicity of milkweed makes both the crawling and flying phases of the insects toxic to predators.
But the Tiger Moth caterpillars go one step further. Tiger Moth Caterpillars literally have “bad hair” and by that we mean poisonous pompadours. Their hairdo includes “urticating” hairs, named after the Latin word for stinging nettle (“urtica”). Some of these defensive growths are long and flexible and some are stout and bristly.
The former can puncture human skin and break off, causing inflammation in sensitive people. The latter can puncture the skin and inject venom (“envenomating” hairs), which can cause a severe reaction in those people. Look, but don’t touch. (Brooklin, Maine)