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This morning, when I wrote the first piece about Black Swallowtail caterpillars, I forgot to mention something rather interesting about their behaviour and appearance. Fortunately, I was reminded this afternoon when my mother, who has been visiting and became involved in the daily caterpillar chores, noticed that the Swallowtail caterpillar “grew horns” when disturbed.

I never knew about Black Swallowtail’s osmeterium — a tentacle-like head gland — until last fall when I happened to disturb a caterpillar concealed in some grass in the perennial garden. It extended an odd looking set of golden antlers and thrashed around a bit — no doubt, trying to give me the “Go away and leave me alone” message. I was unable to photograph it that day, but this little caterpillar seemed quite eager to display this gland, although it looked more like stubby cow horns. It’s not a full-sized caterpillar, so I assume the osmeterium will become much larger by the time it pupates. The gland is supposed to exude an unpleasant odor that wards off some predators. I didn’t notice any particular odor, but one can only hope that it would be enough to put off an Assassin or Stink Bug.

Edit: Wayne from Niches posted a comment about other species of swallowtail caterpillars which he has photographed at Sparkleberry Springs. There are some wonderful photos of the caterpillars with and without their “headgear” on display. These are of Pipevine Swallowtails (Battus philenor), and these are of Giant Swallowtail (Papilio cresphontes). Definitely take time to have a look at Wayne’s pages and photos. By the way, I should probably note that in our posts, Wayne and I have used different spelling of the term used to describe the fleshy gland which can be extended from the head area of a caterpillar. Both osmeterium and osmetrium seem to be in common use.

Tags: Black Swallowtail caterpillar, Papilio polyxenes, osmeterium, osmetrium