Animals as well as humans fall victim to processionaries. Veterinarians are accustomed to treating curious pets that molest caterpillar processions or nests; the unfortunate animals often suffer necrosis of their tongues, requiring that affected parts be cut out to save their lives. In Australia, the processionary Ochrogaster is the prime suspect in a recent rash of aborted foals: there is growing evidence that if pregnant mares ingest fragments of hairs left behind as the caterpillars march over the ground, their fetuses may die. Although the exact mechanism remains uncertain, the hairs may irritate the mare’s gut, which allows pathogenic bacteria to invade the bloodstream.
While it is mostly because of their impact on human health that processionaries have attracted the attention of scientists, I was drawn to them for a wholly different reason. My interest was piqued after reading a series of essays on the pine processionary caterpillar (Thaumetopoea pityocampa) written more than a hundred years ago by the renowned French naturalist Jean-Henri Fabre. Fabre conducted remarkably detailed studies of the larva and the moth, which he recorded in his encyclopedic ten-volume Souvenirs Entomologiques.
In January 1896, Fabre wondered what would happen if the first caterpillar in a procession could be made to follow the last, creating a complete circle. He soon had his answer, for by chance a procession crawled onto a palm pot in his greenhouse and formed a circular procession around its rim. To his amazement the caterpillars circled for seven days before breaking free. Factoring in rest breaks during the cold nights, Fabre calculated, conservatively, that the caterpillars marched for eighty-four hours, circling the rim 335 times. He attributed the circling behavior to blind instinct, stating that the caterpillars lacked “the rudimentary glimmers of reason that would advise them to abandon it.” His account of the circling procession is one of the best known of all insect stories, because it is viewed as a metaphor for mindless living. The story has been endlessly retold by inspirational speakers who see in it the folly of blindly following the crowd, striking off with neither a goal nor a leader, or confusing activity with progress.
I pondered Fabre’s account, not from the point of view of one searching for inspiration, but from that of a scientist who had spent nearly all of?his working years investigating the behavioral and chemical ecology of social caterpillars. In truth, I doubted that caterpillars could be endlessly trapped in a circular procession merely because they adhered to an instinct to follow each other; something else was at work here. I also wondered, as had Fabre, how the caterpillars managed to form and maintain processions in the first place. Fabre observed that each caterpillar lays down a fine thread of silk as it marches along. Although he never formally tested his hypothesis, he felt that the caterpillars sensed those strands, leading them to trail one behind the other. Fabre was an acute observer, but he died long before the discovery of the role of pheromones in orchestrating the collective behavior of social insects. My research with other species of social caterpillars suggested that a previously undetected pheromone might be essential to the formation of processions. Thus, I set off on a study of the behavioral ecology of the pine processionary, which lives in southern Europe and northern Africa.
When I initiated my investigation, I was fully aware of the caterpillar’s toxic nature. Insatiable experimenter that he was, Fabre reported that he suffered severe rashes when he poured extracts of the caterpillar onto his skin. One of his contemporaries, attempting a similar experiment, experienced a much more dangerous anaphylactic response and reported that “not only my hands, my arms, my legs, but my whole body became the seat of insupportable itching; soon my face swelled, my eyes puffed up and I had to give up writing my remarks.” After suffering a severe conjunctivitis when a tiny fragment of a caterpillar’s hair fell onto my eye, I found it necessary to move with caution in the field and to confine the caterpillars in my laboratory to a room fitted with air filters. Nonetheless, there were few days during my studies when I didn’t have to deal with an itching dermatitis.
I conducted my field studies in Catalonia, Spain, where the life cycle of the pine processionary begins in early August, when the moth lays up to 300 or so eggs on pine needles. Soon after they hatch, the tiny caterpillars construct a flimsy silk nest around a few pine needles. That nest is abandoned after a short while, and over the next month the caterpillars collectively build a succession of nests at new sites in the branches of the tree. Their nomadic nesting habits end after their second molt, when they initiate the construction of a dense and virtually impenetrable silk nest they will inhabit for the rest of their larval lives. The permanent nest stands apart from the caterpillar’s food supply, and they march from it to feeding sites on the host tree, returning home hours later with full guts. It is with the initiation of the permanent nest and the long marches that the caterpillars’ hairs, until now soft and harmless, grow into stiff, toxic bristles. Their nest becomes littered with those bristles, fortifying it against attack by would-be predators.