This is a story of the connections between three very different kinds of arthropods, brought about by the annual Northwest Scientific Association meeting, which took place in Olympia, Washington. As usual when a bunch of biologists get together, the final day was a smorgasbord of field trips, and I opted for a jaunt to an acidic peatland. Washington ranks about 21st among US states in extent of peat soils; most of the state’s Sphagnum bogs are in the Puget Sound lowlands, so this seemed like an opportunity that shouldn’t be missed. It was too early in the year to expect much insect life, and the sunny weather that taunted us on indoor presentation days turned cloudy and drizzly, but I tromped out undeterred with bog boots, waterproof camera, and a mild feeling of hopefulness.
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.
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.
(left) Acidic Sphagnum bog near Shelton, Washington (right) Promecognathus crassus, looking good and smelling bad. Photos by C.A. Searles Mazzacano, all rights reserved.
Promecognathines represent an old lineage that probably originated in Pangea; today they are found only in western North America (two species) and the Cape Province of South Africa (six species; [Bousquet, Y. 2012. Catalogue of Geadephaga (Coleoptera, Adephaga) of America, north of Mexico. ZooKeys 245: 1-1722]). The North American Promecognathus crassus and P. laevissimus look quite similar and Yves (2012) doubts they are separate species, but for now based on location I believe my specimen is P. crassus.
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.
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.
(left) A mating pair of Clown Millipedes (Harpaphe haydeniana) in the California Redwoods (right) Parnassius clodius nectaring, Oregon Caves National Monument, Cave Junction, Oregon. Photos by C.A. Searles Mazzacano, all rights reserved.
This remarkable story of evolution and adaptation has an additional player—the butterfly Parnassius clodius (Clodius Parnassian). Parnassians are lovely butterflies with creamy white wings dotted with cranberry-red spots and ornamented by black bars and shading. P. clodius caterpillars aren’t showy but they do come in two color morphs; one is a gray or purplish-brown with light yellow spots along the sides that may merge to form a thin line, and thin black chevrons along the top; and the other is black, with a row of bright yellow spots along each side (McCorkle & Hammond, 1985, Observations on the Biology of Parnassius clodius (Papilionidae) in the Pacific Northwest, Journal of the Lepidopterists’ Society 39(3): 156-162). McCorkle & Hammond note that the grayish form is more abundant in alpine populations of P. clodius, where lighter coloration may help them blend into the rocks, while the yellow-spotted black form is more abundant in lowland populations—places where the Clown Millipede can also be found in large numbers. This suggests the caterpillars are mimicking the appearance of the toxic millipedes in the habitats where they co-exist.
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?
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?
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