In Defense of Caterpillars

A swallowtail caterpillar everting its osmeterium.

Here’s an immature form of the swallowtails we saw in the last post.  Late instar swallowtail caterpillars like the one above have aposematic coloring, warning of the toxic chemicals they have gradually sequestered from the plants they eat.  Swallowtail caterpillars also have another unique defense mechanism–an organ called the osmeterium, located in the head.  When threatened, caterpillars evert the osmeterium, which resembles two brightly colored fleshy ‘horns’ on the caterpillar’s head.  The osmeterium emits foul-smelling terpenes which, along with the startling appearance and the caterpillar’s thrashing movements, helps to ward off predators.