“I must tell you what happened to me on the banks of the Cam in my early entomological days. Under a piece of bark I found two Carabi (I forget which) & caught one in each hand, when lo & behold I saw a sacred Panagæus crux major; I could not bear to give up either of my Carabi, & to lose Panagæus was out of the question, so that in despair I gently seized one of the Carabi between my teeth, when to my unspeakable disgust & pain the little inconsiderate beast squirted his acid down my throat & I lost both Carabi & Panagæus!”
-Charles Darwin, in an undated letter
Emerging from a rotted log, the beetle scurries across the path, its purple-black carapace agleam in the woodland gloaming. The waning daylight is its signal to grow active and alert—time to get up and start the night’s work. It is a ground beetle, Scaphinotus angusticollis, and its morphology attests to a lifestyle most terrestrial: long, nimble legs to hold the body high off the soil, and fused elytra, or wing covers, precluding even rudimentary flight. The beetle’s narrow head, sharp, tearing mandibles, and nocturnal propensities are further clues to its adaptive lifestyle. Darting to and fro across the forest floor, it searches for the telltale trails of slime that lead directly to its unctuous prey: slugs and snails.
Carabidae, the ground beetle family, comprises more than 40,000 species worldwide. Most of these beetles are flightless; many sport black or metallic hues. They are fleet-footed predators, hunting by day and by night. Almost all ground beetles possess structures in the abdomen called pygidial glands that produce noxious chemicals; these are variously exuded, sprayed, or even combusted—such as in the bombardier beetle—to deter predation. The Scaphinotus beetle emits a sickly-sweet odor when disturbed, a smell probably familiar to the gardener who encourages their presence in snail-infested lettuce rows and flowerbeds.
The Carabidae belong to the insect order Coleoptera (from the Greek koleos, meaning “sheath”, and pteron, “wing”, alluding to the elytra-clad wings), also known as the beetles. Coleoptera boasts more species than any other order—around 400,000, nearly forty percent of all known insect species—and its members represent a quarter of all life on earth. Of mammals and birds there are less than 10,000 species a piece. The earliest beetles date back to the Permian Period 290 million years ago, and some estimate that the total beetle species on this planet—both extant and extinct—number in the millions. As the British biologist J.B.S. Haldane once remarked, on making conclusions about life’s diversity from a theological standpoint, “A creator, if one exists, must have been inordinately fond of beetles.”
Suffice it to say, beetles are a big deal. Theirs is a success story par excellence. In their strange, scuttling way, beetles serve as testament to the mutability of life, their countless guises only exemplifying the adaptiveness necessary to flourish on earth. Scaphinotus evolved to tear snails from their shells, but the European-introduced garden slug has proved to be an adequate substitution. Slimy mollusks, shelled or unshelled—it makes little difference to the ground beetle.