It was Wednesday morning and I was already running late to my kinesiology class. The thermometer stood at a fairly frigid (for a Texan) thirty-seven degrees. As I made my way across the campus by way of a sidewalk, my eyes fell upon a shiny purplish-copper-black beetle that was lying torpid on the pavement. I had very few specimens of the ground beetle Diplocheila striatopunctata, which unfortunately goes by no simpler name. Ever the opportunist when it comes to my coleopteran collection, I picked it up, placed it in the top front pocket of my jacket, and proceeded to class. Everything went well until the sudden indoor temperature spike activated the ectothermic beetle’s system, and it began to move. As it did, it struggled against the confines of the wrinkled jacket pocket, and must have come to the conclusion that it had been trapped by some malevolent beast, for indeed it had. A noxious acrid odor suddenly burst forth, emanating from somewhere just below the realm of my nose, permeating the air around me until it was almost unbearable. While I am no stranger to the chemical defensive habits of the beetles in the family Carabidae, I could not recollect the odor from this species being so strong. My nose instantly began to burn, followed by a watering of the eyes. Absent-mindedly I reached my hand into my jacket to make sure the offending insect was still inside, and upon doing so succeeded in gathering a generous acidic coating of the same chemical warfare on my fingertips. Of course, when I pulled my hand out the smell in the immediate vicinity intensified, transferring itself to my pencil, book, and everything else it came into contact with. Noses began to curl. Confused, offended murmurs were exchanged between neighboring students within the fall-out range of the beetle bomb. No one could seem to locate the source, at least not initially, although the girl sitting next to me seemed to be putting the pieces together. Whatever this foreign, blisteringly potent foul cocktail was, it seemed to be emanating from the long-haired nerd with the beetle tattoos on his forearms. A nervous glance occasionally shifted in my direction, suggesting the fear of contagion, followed by an intentionally casual, gradual sliding away of the chairs to the left and right of me. So what was I to do? I suppose I could have excused myself from the classroom and disposed of the beetle, but it was a fine specimen and I didn’t see much sense in sacrificing it due to my ostracism by a classroom of college kids. I could have taken the beetle out and explained the situation to those to whom it might concern, but in all honesty I was having too much fun. The exotic, highly disturbing defensive spray was so unusual, so alien and unidentifiable to my peers that keeping them paranoid and guessing was a sudden tyrannical impulse I could not resist. It may suffice to mention at this point that I am not easily embarrassed. So the beetle stayed, the classroom stank, and everyone patiently awaited the clock.
The beetle family Carabidae is a huge, diverse group of beetles, with 40,000 known species worldwide, 2,000 of which are native to North America, ranking them among the ten largest animal families on earth. They are habitat generalists, occupying almost every type of biome within their vast range except the harshest of deserts.
Ground beetles are, with few exceptions, predatory insects, preying on a variety of invertebrates. This makes them beneficial to agriculture, as their general abundance, coupled with their voracity, aid man in the elimination of such crop pests as army worms and grasshoppers. Other species, such as the boat-backed ground beetle (Scaphinotus), are specialized feeders of snails and berries.
Ground beetles come in a variety of colors, shapes, and sizes, from the tiny Chlaenius, to the multi-hued, shiny metallic jewel known as the fiery searcher (Calosoma scrutator). Others, such as the fighting ground beetle (Pasimachus viridans) possess oversized formidable mandibles that, as I can also attest personally, can deliver a painful nip. The black caterpillar hunter (Calosoma sayi) is another coleopteran gem, bedecked in a densely punctate, grooved coppery metallic elytra aligned with symmetrical rows of copper-colored dots that appear to have been applied by a fine tip brush.
One thing almost all ground beetles have in common is their chemical defense system, which the lowly Diplocheila revealed to my kinesiology class. While this varies by species in chemical composition and degree of potency, these living stink bombs are well-known by entomologists. When disturbed by a predator (or seized between two fingers by an unscrupulous insect collector) a combination of exocrine gland secretions in the lower back of the abdomen jet out a fine mist of an atrocious concoction through a pair of glands called pygidials. This almost always grants the beetle its immediate release. These secretions vary greatly in toxicity, from noxious to caustic. One species in particular, the bombardier beetle (Brachinus sp.) uses a combination of rapidly decomposing hydrogen peroxide and water converted to oxygen through a boiling process, causing hydroquinone molecules in the beetle’s abdomen to oxidize into benzoquiones. The result is an irritating substance that can literally cause mild chemical burns on human skin, and can even stun, blind, or kill lizards, frogs, and small mammals.
The number of different chemical compounds used among the multitudinous species of ground beetles is astonishing. In the genus Calosoma alone researchers have identified eleven different ones, including methacrylic acid, salicylaldehyde, and propanoic acid.
Perhaps no one better to testify to the validity of this defensive arsenal than one James Smith, a coworker and personal friend of mine who also happens to be a mildly interested insect observer and beer connoisseur. On one particularly indulgent night of connisseuring, he took it upon himself to go where no entomologist, professional or amateur, has gone before. He taste-tested a Calosoma scrutator.
The experiment started and ended in a few befouled moments beneath Jimmy’s porch light on a hot summer night. He had been entertaining friends around the cooler when a conversation-pausing resonant buzzing hummed into the midst of their circle like an enemy fighter jet, landing with a distinctive splat against the wall to his right. Without further ado, Jimmy seized his chance. He closed his fingers around the beetle before it could scamper away and popped it into his mouth before his better judgment could convince him otherwise. It was a tragic mistake as the entire interior of his mouth went numb, followed by a bitter, choking taste that drowned out all other thought except to retch and spit and contain the urge to gnaw the very arm of the lawn chair he was sitting in. The taste lingered for hours, and no amount of his friend’s recommended fermented hop wash treatment could subdue it, although he “tried his damnedest.”
A similar incident was recorded by the late, great evolutionary biologist Charles Darwin, who was also an obsessive beetle collector. In a letter of correspondence between him and Leonard Jenyns in 1846, he wrote:
“…under a piece of bark I found two Carabi and caught one in each hand, when lo and behold I saw a sacred Panageus crux major; I could not bear to give up either of my Carabi, and to lose Panageas was out of the question, so that in despair I gently seized one of the Carabi between my teeth, when to my unspeakable disgust and pain the little inconsiderate beast squirted his acid down my throat and I lost both Carabi and Panageas!”
Ground beetles are among the most common of insects, and thus some of the most easily studied. Their color spectrum-encompassing coloration and depth of diversity make them favorites of many coleopterists, and their stunning defensive chemical concoctions, delivered in the form of bombs, gases and blinding sprays, have long intrigued chemists and surprised first-time collectors. I know a certain kinesiology class that will never be the same.
Capinera, J.L. Encyclopedia of Entomology (online)
Eaton, E.R., and Kaufman, K. 2007. Kaufman Field Guide to Insects of North America. Boston: Houghton Mifflin
Entomologist’s Diary. The Little Bombardier. www.entomologistsdiaryblog.com
Evans, A.V. 2014. Beetles of Eastern North America. Princeton University Press.
Linda Hall Library. Scientist of the Day-Leonard Jenyns. http://www.lindahall.org/leonard-jenyns/