The Case of the Vanishing Caterpillar

A butterfly larva’s fate depends on who finds it first—its ant friends or ant foes.

The cycle is renewed as two metalmarks mate. The female will lay the eggs on the underside of a croton leaf. If the resultant caterpillars are not found and hidden by wood ants, they are frequently destroyed by stinging ants and other predatory insects.

Twenty-four years ago, I was a graduate student researching ecology in the Sierra de Tuxtla, an isolated volcanic range that rises from the Gulf coastal plain of Mexico, one hundred miles southeast of Veracruz. One remote habitat in the Sierra especially intrigued me because biologists had never noticed it. This was a small pine community found only on several of the steep ridges radiating down the southeastern slope of Volcán Santa Marta, a dormant, 5,250-foot-high volcano. There I discovered a new species of metalmark butterfly (lepidopterist Harry Clench of the Carnegie Museum honored me in 1962 by naming it Ross’s metalmark, Anatole rossi).

During the first month of my work in the tropics, when I began unraveling the metalmark’s life history, some of the particulars came easily. I rapidly learned that the butterflies live in colonies and that females lay their eggs only on a particular species of croton, a ground-cover plant common in the region. I even discovered a few young larvae, or caterpillars. But then my luck ran out. Even though I found many severely damaged croton plants—they had apparently been eaten by mature caterpillars—I couldn’t find a single large caterpillar or a chrysalis, the developmental stage ii which the caterpillar metamorphoses into a butterfly. Where were the culprits?

I spent my days crawling about on hands and knees, often in rough terrain, searching for large caterpillars. Not finding any, I became very discouraged and resorted to hatching butterfly eggs and rearing the young larvae myself. My combination home and laboratory was a thatch-roofed, mud-floored hut, provided by my hosts, John and Royce Lind, American missionary-linguists who were studying the local Popoluca Indian culture.

Reared in plastic sandwich-box nurseries, the young green larvae grew into large, plump caterpillars, shedding their skins six times in the process. After the second molt, each caterpillar began to secrete droplets of clear fluid from two openings on its back, near the hind end. Near its front end a pair of tentaclelike organs were periodically protruded and withdrawn. In addition, a pair of hard, bladelike structures, which constantly vibrated, projected over the caterpillar’s head. At the time I hadn’t a clue as to the functions of these organs, but my in-house experiments at least proved that the tiny larvae did grow into larger versions. Why couldn’t I find any in the field?

One day, as I sat on a log in the midst of one of my metalmark colonies, nibbling a peanut butter and jelly sandwich (my lunch each day for several months) and pondering the mysterious absence of caterpillars, my reverie was disturbed by motion on a croton plant just three feet from where I sat. I dropped to the ground and drew closer. There, to my surprise, was a fully mature caterpillar, which I recognized from my “nursery” research.

Two Ross’s metalmark caterpillars, accompanied by wood ants, emerge from the base of a croton plant.

Swarming over it were several large, reddish black ants. My protective instinct moved me to rescue the caterpillar from its predicament, but then I noticed that the ants were not attacking the caterpillar, which was feeding docilely; they were probing it, caressing it with their antennae. The caterpillar was protruding its tentacles in response, while the ants climbed aboard and drank droplets of the clear liquid that it released at its hind end.

The caterpillar then began crawling down the plant, and within moments, it was in an underground chamber at the shrub’s base—something I had never seen before. The ants followed, some hitching rides on the caterpillar’s back. Once they were all inside the chamber, the ants closed it with dirt pellets, and all vestiges of insect infestation disappeared. My adrenalin soared! I quickly checked some partly defoliated crotons nearby. Adjacent to each plant was a subterranean niche containing from one to three caterpillars—large caterpillars—and ants; some of the chambers even contained chrysalises. Here was the solution to the mystery, and it had come about so unexpectedly. Although I’ve spent many years observing butterflies, caterpillars, and ants, I have never again seen these ants and caterpillars above ground during the day. They come out from their hiding places only at night. Had it not been for’ my chance encounter with what I like to think of as a gluttonous caterpillar sneaking in an extra meal, I probably would have given up the project of completing the metalmark butterfly’s life history.

Following this revelation, I spent most of my hours—now at night, as well as during the day—on the pine ridges observing the caterpillars and their ant attendants. At night I used a flashlight covered with red cellophane, since my lantern, like any white light that the insects could interpret as dawn, disturbed the insects’ activity. Even my camera’s electronic flash had to be used sparingly and on intermittent nights in order to maintain nearly normal field conditions.

One of two honeydew-secreting glands projects from a caterpillar’s hind end. An ant stroking the glands with its antennae will be rewarded with a sweet droplet.

My nighttime excursions into the forest with camera, tripod, flashlight, and writing paraphernalia, sometimes during torrential thunderstorms, aroused the curiosity of the Popoluca villagers, whose large repertoire of folklore regarding “evils” of the night discourages nocturnal ventures. Within a short time I became the talk of the community, and speculation about my activities reached my American hosts. I learned that the most common theory about my wanderings was that I was searching for lost Spanish treasure, which the Popolucas believe to be buried in their area. At first, this reaction elicited only a chuckle from me, but then the situation became serious. One day I noticed that someone had been digging in one of my colonies and had actually destroyed several of my study plants and their underground inhabitants. Since the Popolucas were unaware of the insects, they paid them no heed as they dug where I had been crouching and peering the previous night. They only wanted to see what treasure I had found.

Following this disaster, my hosts and I met with several of the village leaders. We assured them I was only a “nature lover,” interested in simple things like ants and grubs on plants, and that if their digging continued, my project would be ruined and I would have to leave the area disappointed. Thereafter, I was not disturbed, although I continued to elicit strange glances from many Popolucas, especially the village witch doctor, who continued to spy on me from the nearby shadows.

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