Excerpts from Jim Conrad's
Naturalist Newsletter

caterpillar of Thoas Swallowtail, PAPILIO THOAS with osmeterium

from the December 27, 2015 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO

Soon after my return to Hacienda Chichen, next to the hut I planted a small tree, in Spanish called Hoja Santa, an important member of the tropical Black Pepper Family, the Piperaceae. Our Hoja Santa Page is at http://www.backyardnature.net/mexnat/piper-au.htm.

One morning I was checking on it when a bird dropping turned up on one of the Hoja Santa's big, fragrant leaves. But then the dropping moved. As I stooped and bent for a very close look, in a flash two orange "antennae" materialized atop the poop's bigger end. The whole thing is shown above.

We've seen something like this before. Here at the Hacienda, in 2011, we encountered the Orange Dog caterpillar, the poop-mimicking larva of the Giant Swallowtail butterfly, Papilio cresphontes. Its page is at http://www.backyardnature.net/yucatan/orangedg.htm.

However, that caterpillar had inhabited an Orange tree, a member of the Citrus Family, while this one was on a member of the unrelated Black Pepper Family, plus this one looked a little different. Caution with the identification was called for.

Even if this turned out to be just another Orange Dog, it was exciting, because back in 2011, after volunteer identifier Bea in Ontario had figured out its name, I'd read that Orange Dogs possess "osmeteria," which are orange or reddish, Y-shaped, eversible glands located just behind the head.

"When attacked by small predators such as ants or spiders," I wrote back then, "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."

I wanted to see an osmeterium, so the next day I'd returned to the Orange tree looking for the Orange Dog, but couldn't find it. The next year, however, up in Texas, we did see osmeteria on the caterpillar of a closely related Black Swallowtail, which you can see down the page at http://www.backyardnature.net/n/a/b-s-cat.htm.

For several days I visited our Hoja Santa caterpillar regularly, sometimes accompanied by tourists on my free walks, the poor Hoja Santa always displaying a few more holes in its leaves. Then one day the caterpillar turned up looking as is shown below:

caterpillar of Thoas Swallowtail, PAPILIO THOAS with old skin

He was bigger, bumpier, more brightly colored, and that white, crumpled, cellophane-like thing was sticking to his rear end. Well, not only do butterfly caterpillars metamorphose into pupa, which metamorphose into adult butterflies, but also caterpillars go through spurts of growth, each time shedding their old "skins." Each ever-larger stage between shedded skins is known as an instar. Clearly, our little friend had just grown into a newer, bigger instar. And he didn't seem to be comfortable with the idea. From time to time he'd jerk violently, and didn't move at all for the rest of the day. However, he still could stick out his osmeteria, at least slightly, as shown below:

caterpillar of Thoas Swallowtail, PAPILIO THOAS, with osmeteria

In that picture, notice the brownish, semitransparent item below the head. That's the old covering of his compound eyes. Notice that the old covering is smaller than the compound eyes he has now. A closer look at this is given below:

caterpillar of Thoas Swallowtail, PAPILIO THOAS, new instar

As of this writing the caterpillar still is there, over a week after first turning up, and now he's entered yet another instar, becoming darker and grayer, more ridged and wrinkled, and developing some extraordinary bluish hues only noticeable up close, as shown beflow:

caterpillar of Thoas Swallowtail, PAPILIO THOAS, last instar

So, our caterpillar has really put on a show for us, but was he really an Orange Dog? In reviewing our Orange Dog Page I was reminded that there are two closely related swallowtail species in our area producing very similar caterpillars, of which one was our earlier identified Orange Dog. Back then I also wrote, "...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, there you go. In 2011 we saw the Orange Dog larva of the Giant Swallowtail butterfly, exactly where he should have been, on an Orange Tree, which is a member of the Citrus Family. Now in 2015 we've seen the larva of the Thoas Swallowtail, PAPILIO THOAS, exactly where he belongs, on a member of the Black Pepper Family.

It's wonderful when things fall together like this.

By the way, the "Black Pepper-Dog" name is one I made up. This Black-Pepper-Family-loving species doesn't occur up north in English-speaking territory so it has no English name. The name is just being consistent with the other name, Orange Dog, which is well established in Orange-tree-growing Florida.