Santa Barbara Museum of Natural History

My name is Rosey, I am the Park Naturalist at Cachuma Lake. One of the visitors sent me these pictures of a beetle she found, somewhere off Paradise Road in the rocks. It was dead. Is it a Rhinoceros Beetle? Hercules Beetle? Something else?

Any information would be helpful and much appreciated. See photos below.

Rosey, Santa Ynez Valley - February 20, 2021

Curator Response

Dear Rosey,

This is a great find! This beetle is a male Pleocoma of the small family Pleocomidae, known as rain beetles. This family belongs to the scarab beetle group more broadly, which includes rhinoceros beetles, hercules beetles, dung beetles, chafers, May and June beetles, and many others.

There are two main reasons this find is special–firstly, the family Pleocomidae is a specialty of the West Coast of North America. The about 25 named species occur only in Washington, Oregon, California, and northern Baja California. I hear rumors that there are a few new rain beetle species still out there waiting to be described!

Secondly, their natural history is utterly remarkable. The larvae, or grubs, live underground for many years (up to about 13!), feeding on roots of various plants. The adults do not emerge from underground until the winter rains we receive here on the West Coast, and when these rains occur, members of a given population all somehow synchronize their emergence on the same night! The males are fully winged (you can see the flight wings peeking out like an untucked t-shirt from under the wing covers, or elytra, in the included photos), while the females are wingless. The latter sit at the top of their burrows and release pheromones during a cold, rainy night that often attract a frenzy of males in a desperate bid to mate with the female. Note the clubbed antennae whose segments can be opened up like pages in a book–these segments have receptors that specialize in sniffing out the female pheromone. These beetles have non-functional mouthparts and digestive tracts and do not feed at all during their adult lives; the males generally live for only a chaotic few hours before they completely exhaust themselves and die. Once the female is mated, she heads back underground and waits for her eggs to develop, which can take a few months. One they're ready, she lays them in a spiral pattern at the bottom of the burrow, and the cycle continues.

Almost certainly this dead male was part of a large emergence of rain beetles that occurred during our recent rains.

Thanks for sharing!

Schlinger Foundation Chair and Curator of Entomology Matthew L. Gimmel, Ph.D.