Excerpts from Jim Conrad's
Naturalist Newsletter

caterpillar of either Giant or Thoas Swallowtail

from the April 17, 2011 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO

Next to the septic-tank hole I'm digging with a shovel each morning for an hour or so, for exercise, stands an orange tree. One morning this week, on a stem of the orange I noticed the amazing caterpillar shown above. The caterpillar, about two inches long (5cm), is remarkable because not only is it well camouflaged as a fresh dropping of bird poop, but also its front end looks like a reptilian or amphibian head, better shown below:

front of caterpillar mimicking lizard or snake head, of either Giant or Thoas Swallowtail

To me the head looks like it belongs to a gecko or other kind of lizard, but others say it's a snake's head. Whoever the head looks like, the fake eyes, nostrils and wide mouth could be disconcerting to any predator approaching from the side or front.

Naturally I sent photos of the caterpillar to Bea in Ontario, who immediately recognized it as a member of the Swallowtail Family, the Papilionidae. Starting with that, it wasn't long before she knew we had one of two species: Either it was the caterpillar of the Giant swallowtail, Papilio antimachus, or else of the very similar Thoas Swallowtail, Papilio thoas. Since we've identified Thoas Swallowtails here and not Giants, we figured that the caterpillar is that of the Thoas Swallowtail.

However, an expert Bea double-checked with in the US thinks that Thoas butterfly caterpillars probably feed on members of the Black Pepper Family, the Piperaceae, while Giant Swallowtail caterpillars feed on citrus, so he's guessing that it's a Giant Swallowtail. So, we're going with that: Our caterpillar is that of the Giant Swallowtail, PAPILIO ANTIMACHUS.

In citrus growing parts of the southern US caterpillars of the Giant Swallowtail are known as Orange Dogs or Orange Puppies, presumable because they feed on leaves of orange trees, doing considerable damage.

When I took the above pictures I didn't know one of the most fascinating facts about this caterpillar. That is, in addition to looking like bird poop and a lizard's head, each larva possesses an "osmeterium," which is an orange or reddish, Y-shaped, eversible gland located just behind the head. When attacked by small predators such as ants or spiders the caterpillar extrudes the gland so that it looks like an orange or reddish antler, and tries to wipe it against the attacker. The osmeteria of older caterpillars contain a highly noxious, pungent chemical that smells like rancid butter.

The day after I took the above pictures I returned to the tree hoping to antagonize the caterpillar into extending osmeterium, but he was gone, or least better hidden than when I'd earlier found it. But you can bet that the next one I see I'll try to see that osmeterium.