An Introduction to Pleocoma Rain Beetles

He’s large. He’s shiny. He’s hairy. Oh, did I mention that he’s super hot? Literally hot: he has an insanely high average body temperature. He can fly (but she can’t), he can crawl. He can do a back flip. He’s a Rain Beetle from the genus Pleocoma. He’s a true beauty of the North American west, found from Washington down to Baja California and east all the way to Utah.

Fig 1 - Male and female rain beetles. Source: Essig Museum, Berkeley

Fig 1 – Male and female rain beetles. Source: Essig Museum, Berkeley

Pleocoma is the only genus of the family Pleocomidae and contains approximately 30 species. Pleocoma are called rain beetles because adults usually come out after autumn or winter rains. They are called Pleocoma from the Greek root “pleos,” meaning “hair.”

Rain beetles typically live quite a long time — they sometimes take up to 13 years just to mature from larval stages to adults! During this time they molt about seven times. They like to feed on the roots of oaks and conifers. That being said, a adult rain beetles don’t have mouthparts so they literally cannot eat. Instead, they utilize all their stored fat that they obtained while feeding as grubs. Obviously, this means that adults will not be active very much, around 2 hours of air time to be exact. They only live several days as adults. Females however require less energy and can live up to several months. Imagine being a child for your whole life only to die shortly after becoming an adult!

Physically, rain beetles are generally large, robust, and appear shiny. They have a thick layer of hair covering the undersides, which act as insulation for males who must maintain high body temperature in places where it is cold. Males can maintain an internal temperature of up to 95 degrees F. The thick layer also functions as a layer of protection against abrasion when they burrow through the soil. Both sexes dig with powerful, rake-like legs and a V-shaped scoop mounted on the front of their heads. Males have full and functional wings capable of flight; on the otherhand, the wings of females are flightless and are merely small flaps of tissue. However, females are generally much larger and more heavy bodied than males.

Fig 2 - Relative size of a rain beetle. Source: U.S. National Park service

Fig 2 – Relative size of a rain beetle. Source: U.S. National Park service

During courtship, males fly low over the ground searching for the females’ pheromones. Males will usually come out after rainfall. They appear suddenly and in huge numbers, looking to mate. Females cannot fly, so they have to wait for a male to find them. Mating is an extremely competitive process; many males will swarm a single female, fighting each other for position. A rain beetle’s entire life leads up to this epic battle for mating! After mating, the female burrows up to ten feet below the ground to lay her eggs. Female rain beetles will usually lay 40 to 50 eggs at once.

In terms of the current state of the rain beetle, local disturbances such as development, logging, mining, and other forms of destruction have had negative effects on the species. Additionally, many domestic predators, such as dogs, racoons, and skunks have been further contributing to the reduction of these beetles.

If you want to catch sight of these amazing insects, they can be found in Berkeley’s Tilden Park the morning after the first fall rain. You can also find them outside at dusk or on a rainy day. Elsewhere, rain beetles can be found throughout the West Coast of the United States, from the state of Washington all the way south to Mexico. Because the rain beetle population is not as widespread as other insects, human activities such as logging and mining have much more negative effects on the rain beetle population. In 1994, the Santa Cruz rain beetle was almost placed on the endangered species list!

Rain beetle hiking through some debris:


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