It's Raining…Rain Beetles?

A male rain beetle. Painting by Jack Laws, from his Naturalist Notebook page on this in our Oct-Dec 2006 issue.

by Kate Brittain — published October 31, 2008

As April showers are to May flowers in other parts of the country, so are autumn rains to the rain beetle here in the Bay Area. The rainy season's first soaking precipitation (an inch or more) beckons forth this remarkable insect.

After ten to twelve years underground--and up to a month of this time spent as adults--the beetles emerge to mate. The winged male of the species has enough energy for about two hours of flight, which he spends searching for a flightless partner, who waits for his arrival at the entrance of her burrow. Keeping low to the ground, the male--aided by a pair of large antennae--follows the scent of the female's pheromone.

Once she's found, the pair mate. The female then closes off the entrance to her burrow and tunnels down to lay her eggs. Adult Rain Beetles have neither functional mouths nor digestive tracts. They have one purpose only--to reproduce--and their anatomy reflects the fact. For the female, this means a larger body with powerful legs and a rigid digging device (called a clypeus) at the end of her head, designed for expedient excavation. Tunnel complete, she lays her eggs in a spiral at its base. They mature the following spring.

If you're hoping to glimpse these ephemeral insects, try Berkeley's Tilden Park the morning after the first fall rain. At dusk, or on a drizzly day, you'll also have a good chance of catching the critters out. Be on the lookout for roughly quarter-sized emergence holes and the males, in fast flight, following their antennae.