The Case of the Vanishing Caterpillar

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

Protuberances over the caterpillar’s head shield two short sword-shaped beaters. These specialized appendages produce faint clicking sounds that attract wood ants.

In the years since my discovery of Ross’s metalmark, I have worked out its life history, which is probably the most complex of any known butterfly species. The butterfly is relatively small, with checkered brown, orange, white, and black markings. Like most other metalmarks around the world, the adults spend most of their time flitting rapidly two to five feet above the ground or clinging to the tops and bottoms of leaves, where they bask in the sun with their wings outstretched. In clear weather the butterflies are particularly visible late in the afternoon, when they dart about erratically and chase each other for short distances in the last rays of the setting sun.

The adult insects feed almost exclusively on the nectar from the white flowers of Croton repens, a wild relative of the more showy crotons raised indoors by many Americans and Europeans. The first indication that the butterfly’s life cycle is unusual is that, while the croton plants are widely distributed, the butterflies congregate in small, isolated colonies. These colonies, which contain eight to fifty butterflies, are found only on certain sunny ridge crests and upper slopes and only in the vicinity of the few human settlements in the area—five small Popoluca Indian villages.

Within the butterfly colonies, the female insects lay their eggs on the undersurfaces of croton leaves. Hidden on the underside of the leaves, the very young, inconspicuous green caterpillars spend their days and nights nibbling the plants’ tissues. Eight to ten days after hatching, a larva develops the specialized structures that I had noticed in my nursery. These enable it to attract its ant associates. A pair of glands near the larva’s hind end produce a sugary secretion known as honeydew, which ants eat with the gusto of children devouring candy. A variety of insects-aphids, many scale insects, and to a lesser degree, certain butterfly and moth caterpillars-produce similar honeydew secretions. The tentaclelike organs near the front of the caterpillar secrete a gaseous chemical pheromone, or scent, which also attracts the ants. And the caterpillar’s vibrating, bladelike structures produce faint clicking sounds, inaudible to humans but within the range of insect perception.

An ant grasps a globule of honeydew in its jaws.

Not long after these specialized organs become active in a newly molted larva, the caterpillar attracts its associates, ants of the species Camponotus abdominalis, commonly referred to as wood ants or carpenter ants because they nest in old logs, fence posts, and even house beams. This species can be found throughout most of tropical Mexico and Central America. At night these ants scour the countryside (and villagers’ kitchens) looking for, above all else, sweet things to eat. Needless to say, the Anatole caterpillars with their honeydew glands fill the bill.

When it first encounters a caterpillar during a nocturnal foray, the ant uses its antennae to stroke the caterpillar in the vicinity of the honeydew glands. The caterpillar responds by secreting tiny drops of moisture, which the ant imbibes. Usually within a few minutes, the ant worker is joined by several others of its kind, all of which consume the seemingly endless supply of treats. The wood ants do the caterpillar no harm, and after some thirty minutes of feasting, depart in relays to dig a small, shallow trench around the base of the croton. This excavation takes several hours but usually is complete by dawn. The resultant circular excavation is one to two inches deep. The ants then swarm about the caterpillar, constantly stroking it with their antennae. The caterpillar is induced to crawl down and soon finds itself underground. The ants quickly seal the entrance to the hole with small pellets of dirt from their previous digging. In a sense, the ant ranchers have herded their “cow” into its pen and closed the gate so that the milking becomes a private affair.

At dusk, between 7:00 and 7:10, the pellets of dirt are removed from the roof of the pen and the ants emerge, temporarily leaving their captive behind. They run back and forth over the leaves of the plant in a manner that suggests a search for potential enemies. To test if this was the case, on several occasions I placed small predatory arthropods, such as bugs and spiders, on the plants. The ants immediately seized these victims with their powerful mandibles, sprayed them from their abdomens with formic acid—an acrid liquid that smells something like vinegar—and carried them down the plant, depositing the virtually lifeless forms on the ground a few inches away. (Incidentally, the Popolucan name for these ants means “the sour-smelling ones.” The villagers’ language is rich with descriptive names for most common plants and animals; however, it doesn’t contain a name for the elusive metalmark caterpillar.)

In a subterranean chamber, a caterpillar attaches itself to a croton stem, sheds its skin, and becomes a chrysalis, or pupa. Although it produces no honeydew, the chrysalis will be tended by the ants for nine days of the eleven-day pupa stage.

After about ten minutes of running over the plant, the ants return to the subterranean enclosure and vigorously stroke the caterpillar with their antennae, apparently coaxing it to move. The caterpillar now ascends the stem, shepherded by its carpenter ants, and eventually settles on a tender leaf near the top of the plant to begin feeding.

The feeding continues intermittently throughout the night, with the ants constantly in attendance, crawling over the caterpillar, pausing occasionally to imbibe droplets of honeydew, and running up and down the croton plant in search of marauders. The caterpillar, meanwhile, frequently emits its pheromone and vibrates its bladelike beaters, causing the ants to become more active and attentive. Between 4:30 and 4:45, just before dawn, the ants herd their charge downward and into the underground chamber. The roof is replaced, and even before the first streaks of light cross the tropical sky, both cow and herders are secure within their corral until the next twilight.

The daily cycle is rarely altered or interrupted during the summer and fall months. Even torrential rains do not have any observable deterrent effect. The underground pens are always located on sloping terrain of heavy red clay that provides excellent drainage, so the pens are rarely flooded. Besides, the insects are on the croton plants during the night, when most of the rains occur.

The caterpillars require nearly two months to complete their development. Then, like all butterfly and moth larvae, they enter a resting and transformational stage called the pupa. Unlike moth larvae, butterfly caterpillars do not encase themselves in cocoons. Instead, they attach their abdomens to stationary objects and their skins to reveal a new form, the chrysalis. Then, one of the greatest marvels of the insect world occurs: the lowly crawler is transformed into a winged gem of the skies.

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