On Trails by Robert Moor

learned tent caterpillars deposit pheromone trails by dissecting ants.
    It may seem odd, then, that neither Wilson nor Fitzgerald citesBonnet’s discovery. In fact, many of Bonnet’s writings, including the story of how he discovered the true nature of ant trails, have never been published in English. Though his career showed a promising start, it ultimately veered off on an ill-fated path. In his twenties, Bonnet became a celebrated naturalist: the first person to witness a virgin birth among plant lice, the first to describe regeneration among worms, the first to learn that caterpillars breathe through holes in their skin, and the first to prove that leaves exhale. Then, in a cruel twist, his vision began to cloud with cataracts. Unable to practice observational science, he turned to more cerebral fields, like philosophy, psychology, metaphysics, and theology. Much of the latter half of his life was spent trying to reconcile the confusing new findings of the biological sciences with his deep religious faith, which held that the world was divinely engineered. Bonnet’s magnum opus—an all-encompassing theory of the universe called the “Great Chain of Being,” which posited that all species were slowly progressing toward a state of perfection over the course of eons—had some influence on later evolutionary theorists like Jean-Baptiste Lamarck and Georges Cuvier. But in the broader span of scientific progress, it proved little more than a theoretical side road, which was later made obsolete by Darwin’s theory of evolution by natural selection. By the end of his life, Bonnet’s blindness caused him to suffer from phantasmagoric visual hallucinations, which are now known as Charles Bonnet syndrome. I Today, that syndrome is primarily what he is remembered for, when he is remembered at all.
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    Every trail tells a story, but some trails tell it more eloquently than others. The trails of Despland’s forest tent caterpillars, for example, are blunt—they are essentially able to shout just one phrase: This way! The trails of certain ant species are more sophisticated: they can whisper as well as shout. The strength of the chemical trail tells the colony how desirable the trail’s destination is, which allows for more nuanced communication and nimbler collective decision making. Scientists have long pondered how ants, which are individually quite stupid, can behave so intelligently as a colony. “The reason is,” E. O. Wilson once wrote, “that much of the ‘spirit of the hive’ is actually invisible—a complex of chemical signals we have only now begun to reveal.”
    Consider the fire ant: Once a scout has found a food source, excited by its discovery, on its return trip it presses its stinger to the ground to release a stream of pheromone, like ink from a fountain pen. The more food it finds, the more pheromone it deposits. II Other ants follow this trail to the food, and then they lay more trails home. So if there is a large store of food, the trail will emerge quickly and blaze bright (chemically speaking), which will attract more ants. Then as long as food remains, the trail will continue to draw more ants. But once the food runs out, the trail evaporates, and the ants gradually abandon it for another, stronger trail. This process neatly illustrates how stigmergy allows simple beings to arrive at elegant solutions to complex problems all on their own.
    The basic mechanism at work here is the feedback loop: cause leads to effect (an ant finds food, and deposits a trail as it returns to the nest), then that effect becomes a new cause (that trail attracts more ants), which then leads to an amplified effect (they lay down their own trails, recruiting more ants), ad infinitum. Feedback loops can be divided into two types: the desirable kind, known as a virtuous circle , such as when ants leave stronger and stronger trails to a food source; or the undesirable