Caterpillar or coral snake?

By Jeff Mitton

I was stalking a black cuckoo called a smooth-billed ani in a botanical garden on the island of Dominica. Creeping around a golden trumpet bush, I saw something that stole my attention. The bush was adorned with brightly colored caterpillars, the largest I have ever seen.

The caterpillars were between five and six inches long and about a half inch in diameter. They were velvety black with bright yellow rings circling the body and orange legs with black dots. The head and mouthparts were orange.

The caterpillars were eating the thick glossy leaves of the golden trumpet and had no reaction when I approached. A feeding caterpillar is a machine in constant motion, its head moving to cut a swath of leaf, biting and swallowing continuously. A lack of any effort to escape convinced me that the brilliant colors were warning coloration, advertisement of an adequate defense. I watched the caterpillars but did not pick them up.

These caterpillars grow into tetrio or frangipani sphinx moths,Pseudosphinx tetrio, with a wingspan of five inches. In contrast to the bright colors of the caterpillar, the tetrio moth is a subdued but intricate pattern of black, grey, white and several shades of brown.

Tetrio sphinx is found from southern Brazil through Central America and Mexico, the Caribbean Islands and southern Florida. Adults range widely and have been documented in the states along the Mississippi River and as far north as Pennsylvania.

Tetrio sphinx caterpillars feed on just a few species of plants. The two favored hosts are golden trumpet,Allamands cathartica, and frangipani,Plumeria rubra. Both species are in the dogbane or milkweed family and are planted as ornamentals. Like milkweeds, golden trumpet has milky sap that irritates the skin on contact. Golden trumpet synthesizes the toxin allamandin, which can be prepared as a purgative that induces both vomiting and diarrhea. The specific name,cathartica,is taken from catharsis, referring to the purging effect of allamandin. It is an effective defense for the caterpillars, most of the time.

The smooth-billed ani is one of the few predators that found a way around the caterpillar's toxic defenses. It slaps the caterpillar against a branch until it breaks apart and then consumes tissues that have little or no allamandin, avoiding the stomach and intestines, which have high levels.

Caterpillars are also protected by urticating hairs with reverse barbs that anchor them in flesh, preventing their removal. Movement drives the hairs deeper, causing a persistent irritation that evokes furious scratching.

Famed tropical biologist Dan Janzen has suggested that tetrio sphinx caterpillars are mimics of poisonous coral snakes, which are brightly marked with black, yellow and orange bands. The caterpillars are not close mimics of any one species, but the superficial resemblance is backed up by a unique bit of theater. Janzen reported that when a person picks up a caterpillar it thrashes its head back and forth and delivers a vicious bite. That would certainly convince me to back off.

The tetrio sphinx has an attractive caterpillar with multiple modes of dissuasion: warning coloration, toxic chemicals from host plants, irritating hairs and the color, pattern and behavior of poisonous snakes.

Jeff Mitton (mitton@colorado.edu) is chair of the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Camera.

May 14, 2010