This is one of the few times I’ve seen this orange gland on an annoyed Black Swallowtail Butterfly caterpillar. Usually, the caterpillars are fairly easy-going and don’t mind me puttering around. The black swallowtail caterpillar has an orange “forked gland”, called the osmeterium. When in danger, the osmeterium, which looks like a snake’s tongue, appears and releases a foul smell to repel predators. I didn’t smell anything, so I’m lucky.
Catherine Sherman 
