Ah, my pretty, you're ... #&! a beetle pile!

Hundreds of tiny, young beetles lump together in the shape of a female bee and then grab rides on the male bees that they seduce, say California researchers.

Watching a male bee snuggling down to what had seemed an inviting partner and discovering himself laden with beetle larvae is "pretty remarkable," says John Hafernik of San Francisco State University. "You can't see terror in a bee's eyes, but you can imagine it."

This faking out of solitary bees by the blister beetle Meloe franciscanus marks the first recorded case of parasitic insects cooperating to mimic their target, report Hafernik and his San Francisco State colleague Leslie Saul-Gershenz. In the May 4 NATURE, they describe Meloe clumps in California's Mojave Desert.

The Meloe "could be an interesting system for studying how cooperative behavior evolves," Hafernik suggests. He notes that beetles don't have the social insects' reputation as intellectuals.

Other researchers had reported aggregations of beetles. However, Hafernik says he hasn't heard anyone interpret the clusters as a way for young beetles to find a home.

The way home for the blister-beetle larvae is via a female bee. As she packs her eggs into sand burrows and provisions them with pollen, beetle larvae slip into the nurseries. After the mother bee flies off, the beetle larvae raid the bee's baby food and assume a grublike form, finally pupating into wingless adults.

Before this study, researchers had assumed that larvae at the orange-louse stage, called triungulins, behave like many related species that labor up a stem to lurk in a flower and grab visiting bees.

However, the California researchers now propose that larval aggregations attract the male Habropoda pallida, a species of solitary bees in the Mojave. Larvae clinging to males later move onto females during mating.

Hafernik and Saul-Gershenz found that beetles typically stay clumped for 5 days but sometimes hold out for 2 weeks. The researchers checked 1,200 flowers near the aggregations but found no larvae.

The team saw 98 instances in which a bee buzzed up to investigate a beetle clump. In 42 cases, the researchers figured out the arrivals' sex. All were male.

The bees didn't act as if they were approaching a flower, Hafernik says. When nearing food, foragers typically stick out their tongues, which extend perhaps half their body length, but the researchers saw no such behavior.

If the bees weren't going to eat the lump, they could easily be considering having sex with it, Hafernik argues. "There are only a couple of things that male bees do," he explains.

The alluring profile of a luscious female and some odor cue emitted by the beetles may be combining to trick the bees, the scientists speculate. Bees ignored painted cellulose models of beetle aggregates and homed in on real beetles.

Beetle specialist Terry L. Erwin of the Smithsonian Institution in Washington, D.C., calls the aggregation "pretty exciting." He's seen larvae of a species of carabid beetle cooperating to attack another beetle's pupa that stands on end using a muscled anchor. The crush of carabids prevents the pupa from flexing to whack them.

The Smithsonian's Ronald J. McGinley greets the work with a flood of excited questions, such as, How in the world does a speck-brained larva know when to grab onto a male bee, transfer to a female, and let go to end up in a burrow?

As a bee specialist, he muses about the proposed mimicry's similarity to pollination of certain Ophrys orchids. Their perfumes seduce male bees into trying to mate with flowers. He finds it plausible that some dim visual cues and possibly odor could likewise befuddle the Mojave species. "Bees do a lot of dumb things, just like humans," McGinley notes.