I was busy chasing a Pardosa wolf spider across a puddle when one of my fellow attendees nudged me and held out his hand. Nestled on his palm was a shiny black beetle with preposterously huge, forward-projecting jaws. As I leaned in for a close shot, I also became aware of a substantial stink wafting from the beetle. I hadn’t seen this critter in the field before, but its general appearance was that of a ground beetle (Carabidae), and upon returning home I determined that it was in the genus Promecognathus.
Ground beetles are a large, diverse family composed mainly of predators. When threatened, some can also generate a potent shield of stink by secreting odoriferous and sometimes caustic compounds from glands near the anus. These organic cocktails of hydrocarbons, ketones, esters, acids, quinones, aldehydes, and phenols pack a startlingly powerful reek. Promecognathus is flightless, with reduced (brachypterous) hindwings, so the best defense for this earthbound beetle is apparently simply to be offensive. But what about those giant projecting jaws, the monstrous mandibles that give the genus its name (Promecognathus is derived from the Greek ‘promeces’, meaning advanced or in front of, and ‘gnathos’, meaning jaw)?
Most adult carabids are opportunistic predators, killing whatever they can catch, but some are specialists, concentrating on caterpillars, snails, or slugs. Promecognathus specializes in polydesmid millipedes. Our west coast Clown Millipede (Harpaphe haydeniana) is commonly encountered and easily recognized, with a large (up to 3 inch) chocolate-brown to black body studded along the sides with bright yellow dots. Clown Millipedes are important nutrient recyclers in forest ecology, munching on and breaking down tree leaves and needles, and they are abundant and slow-moving, so potentially a great snack for a predator. However, they have their own chemical weapon that ups the ante on bad-smelling beetles—they produce hydrogen cyanide (HCN) and benzaldehyde. HCN is a potent metabolic toxin that makes the millipedes extremely poisonous to small animals (but imparts a pleasant smell of almonds) and renders them almost immune to predation—except for Promecognathus. Using their long legs, adult beetles straddle a millipede and run up to its head, then sink those remarkable mandibles into its neck, severing the millipede’s ventral nerve cord and thus preventing it from using its cyanide defense spray.
An edible mimic of a toxic model (a.k.a. Batesian mimicry) is only protected as long as predators are more likely to come into contact with the noxious model and have an unpleasant learning experience that results in future avoidance. Where edible mimics outnumber noxious models, Batesian mimicry can be a failure for the mimic, but the greater abundance and availability of millipedes vs. caterpillars works in favor of the caterpillar. There is also some question as to whether the caterpillars might themselves be distasteful, since this species uses Dicentra (bleeding heart, Dutchman’s Britches) as larval host plants. Dicentra contain toxic alkaloids (James & Nunnallee, 2011, Life Histories of Cascadia Butterflies, OSU Press), and a rather horrifying paper by Black & Kelly, published in 1930 when animal use standards were presumably different, describes in detail the death convulsions of multiple white mice injected with different concentrations of Dicentra extract (Toxicity of Bikukulla formosa, Western Bleedingheart, Journal of Agricultural Research, 40(10): 917-920).
Who knew that a peatbog Promecognathus would reveal such stirring biological drama?