All swallowtail butterfly caterpillars in Michigan, regardless of species, have a unique defensive mechanism called osmeterium. When disturbed, the caterpillar is able to stick out and retract little horns at the top of their head. While the little orangish colored horns may themselves surprise a potential predator, the smell is what is really noticeable. I would describe it as a odiferous sweet smell. Many people describe as stinky and use the name “stink horns”. It’s never really bothered me. In fact, when I have been caterpillar hunting, it has often aided me in finding them, because the smell is so unmistakable. I know a swallowtail caterpillar is nearby when I smell it. This a a tiger swallowtail caterpillar with it’s osmeterium or “stink horns” out. When they stick them out they extend further than this but usually quickly retract much of it and then shortly after pull them entirely back in.