| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Wild Lives Bungling Beetles Unless theyre as hungry as locusts or as deadly as killer bees, few insects ever make the headlines. But in my neck of the woods, theres an exception. Each fall, after the first good rain has soaked the soil, male Pleocoma beetles make their appearance. And when these shaggy, dark insects the size of jumbo olives thud against your window at night, they get your attention. By the time our Nevada County newspaper ran the front-page headline “Beetles spook residents” the day before Halloween last year, Id already spent a couple of evenings watching these awkward insects cruising slowly through the woods near my house. That morning, in fact, Id discovered two lustful males half-buried in a head-to-head impasse in my garden. Apparently, theyd reached the burrow of their dream-mate at the same moment and in their excitement to dig to her earthen cell, they were blocking each others path. Unable or unwilling to back up, theyd been locked in this standoff all day. Bumbling reproductive behavior is just one of the eccentric features of rain beetles, the common name for insects in the genus Pleocoma. From their extraordinary longevity, to the sizzling metabolism that fuels their final moments, these insects are an odd lot. The genus comprises some 30 species, scattered throughout the wooded hills and mountains between northern Baja California and southern Washington. (Records of three species in Alaska and Utah have been proven wrong.) As could be expected of a group of subterranean insects with flightless females, no species is widely distributed. Many are limited to colonies that occupy scattered sites within their range, and the ranges of some are no more than one or two counties. But within a territory, they are abundant. Dig a refrigerator-sized hole in Pleocoma habitat and youll likely find between a handful to a few dozen of the wormlike, C-shaped larvae. The common, flightless ancestor of the genus was probably widespread before the beginning of the Miocene epoch, 24 million years ago. Californias mountains were lower and gentler then, posing less of a barrier to an animal whose only means of expanding its range is by chewing its way through the soil. Even today, two million years after Californias inland seas dried up, the waters effect on the beetles distribution is striking. Wherever water covered the state then — including much of the San Joaquin Valley and northern portions of the Coast Ranges—there are no Pleocoma now. Rain beetles are scarabs, that family of large-bodied beetles that includes the dung beetle, an insect held sacred by ancient Egyptians. Pleocoma translates from Latin and Greek as “many hairs,” and rain beetles are indeed hairy. Long bristles cover their undersides, hang from their legs, and line their wing covers. Adult beetles are broadly oval and shiny, varying in color from dark honey to mahogany to black. Males are somewhat smaller than the one- to two-inch-long females. Perhaps my Nevada County neighbors would have felt more kindly toward the beetles that invaded their yards last year if theyd known that Pleocoma lead a long but—from a human point of view, at least—miserable life. By the time they mature, Pleocoma have spent up to twelve years underground as larvae (also called white grubs), subsisting on the tough woody roots of trees and shrubs. Only a handful of insects are known to live longer. Unlike most other soil-dwelling grubs, which dig with their six thoracic feet, Pleocoma use powerful mandibles to move through the soil one bite at a time, pausing between every couple of mouthfuls to deposit the excavated dirt into the tunnel behind them, deftly maneuvering the clods with their mouthparts and legs. On a good day, a grub in top form can move two to three inches through hard-packed soil. Given the rigors of grub life, its no wonder that after a decade in the soil, the insects manage to lay up only a tiny amount of body fat. The fat, however, is as precious to the beetles as rocket fuel to astronauts. Why? Because adult Pleocoma cant eat. During pupation—that wondrous transformation from fleshy larva to hard-bodied adult—a rain beetles mouthparts fuse. This poses no problem for earth-bound, swollen-bodied females; their caloric requirement is low. But male beetles wander about in the cold. The first heavy autumn rains trigger their emergence from the soil. These hardy creatures have even been spotted darting through snowflakes. Chillier still, most Pleocoma fly during the coldest hours: at dusk, at dawn, or at night. And heres the biological rub: In order to fly, heavy-bodied insects must be warm. To fly fast, they must be hot. Fueled by a metabolic reaction that uses stored body fat, the thoracic temperature of a flying or crawling Pleocoma can reach 95 oF, even in freezing weather. But the energy doesnt come cheap. The biologist who studied rain beetle metabolism, Kenneth Morgan, calculated that from the moment a rain beetle first spreads its wings, it has only enough fat for two hours of flight. When the fat runs out, the beetle dies. Perhaps this is why female rain beetles emit a lemony odor so strong that people can smell it and dogs drop to the ground to roll in it. This attractant (presumably a pheromone) produces a sweeping effect thats unusual in the insect world. Male rain beetles heed the scent of female rain beetles, regardless of species. Hunting for the perfume, male Pleocoma jag through the woods, flying low and slow like heavy-laden bombers searching for their target. (This modest pace makes them easy marks for birds and even mammals. Taking short leaps, agile foxes and coyotes snag the beetles right out of the air.) Once he homes in on the odor wafting from a females burrow, a male beetle drops to earth and scrambles over the ground at random. If he finds the opening to his sweethearts tunnel, he enters, digs his way down, mates, then just as quickly hurries away in the hope of finding another mate before running out of fuel. Up to eight months after mating, females lay several dozen fertilized eggs in their burrows. Then they, too, die. In the summer, eggs hatch into larvae and the life cycle begins anew. For two days last fall I watched the battle of the beetles in my yard. But the dueling Pleocoma remained frozen in the position Id found them in—half-buried in a females burrow, butts pointed skyward. So near, yet so far. It was a skunk, most likely, that sealed their fate. On the third morning, all that was left of the combatants was a couple of wing covers. Given their propensity for disaster, its comforting to know that about twice as many males as females hatch from Pleocoma eggs. Though evolution has cast these insects a hard lot, its done what it can to ensure survival of the species. Liese Greensfelder is a freelance writer in Nevada County, California. She thanks beetle specialist Frank Hovore for his genial help with this story. |
Fall 2000
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