A MILLIPEDE’S BRISTLING DEFENSE
While peeling back some loose bark on pine stumps and logs at Hughlett Point Marsh Natural Area Preserve in Northumberland County, Virginia, I happened across a small assemblage of bristly and segmented animals measuring no more than a few millimeters.
Across their backs were bands of short, stiff bristles and their sides were festooned with blooms of even more spikes. And projecting from their rear ends were pairs of brushy tufts. These intricately adorned and prickly animals were bristle millipedes, Polyxenus fasciculatus, a species widespread in drier habitats throughout eastern North America.
Most millipedes protect themselves by exuding noxious chemicals from a series of pores along the sides of their bodies to deter predators. Depending on the species, these millipede secretions contain alkaloids, benzoquinones, cyanogenic compounds, phenols, or quinazolinones.
But you don’t have to be a biochemist to fully appreciate the impact of these compounds on the appetites of even the hungriest of predators. Anyone who has ever handled any of these millipedes and smelled their sometimes stained fingers afterwards can easily attest to the fact that these slow-moving animals would indeed be tough to swallow.
In spite of the formidable defenses provided by these chemical compounds, bristle millipedes rely instead on purely mechanical means to protect themselves. Chemical ecologist Thomas Eisner and his colleagues at Cornell University ably demonstrated that bristle millipedes use the detachable bristles from the brushy tail tufts to immobilize their enemies, such as ants, centipedes, spiders, and pseudoscorpions.
The tail tufts are made up of individual bristles with barbed shafts and tipped with several hook-like prongs. When attacked, the millipede splays out its tufts and dabs them against its antagonist, sometimes several times in quick succession.
As the hooks snag the ant’s own body bristles, antennae, mouthparts, and feet, the millipede’s bristles easily detach from the tufts. Attempting to free themselves of bristles, hapless ants simply disperse the “sticky” bristles on their bodies even further. And their frantic struggling only serves to interlock the barbs on bristle shafts and create an inescapable mesh. Once nimble ants quickly find themselves completely hog-tied. With no hope of escape, heavily entangled ants become immobilized and soon die.
Since millipedes grow and molt throughout their lives, bristles lost in defensive actions are replaced with each molt. In fact, it just may be that the loss of bristles is what triggers the physiological changes necessary to initiate the next molt.
The original research conducted by Eisner and his colleagues, with magnified images of bristles, is available at: http://www.pnas.org/cgi/reprint/93/20/10848
©2008, A.V. Evans