I apologize in advance for putting up an ARKive video that is self-starting (they all are from that site), but thank Ceiling Cat it is relatively silent. And it’s worth the watch.
I learned about this phenomenon from reading The Folly of Fools, Bob Trivers’s new book on deceit and self-deception, and have read the paper in Nature (reference below) that describes it. It’s a case in which a group of larval beetles work together to imitate a bee, fooling a real male bee into copulating with the mass of larvae, who then are transferred to female bees via real bee-to-bee copulation, and then taken to the female’s nest, where they spend the next stage of their life.
According to the paper by Hafernik and Saul-Gershenz, the larvae of the blister beetle Meloe franciscanus emerge from the sand in the Mojave Desert and immediately aggregate, crawling as a mass onto vegetation. They then form a bee-sized ball that “responded to outside stimuli, such as nearby movements, by waving their front legs or by contracting as a unit.”
You can see all this on the video below.
This moving ball of blister beetles beckons to male bees, who think it’s a female and try to copulate with it. That’s also on the video. The researchers watched 42 bees of the species Habropoda pallida approach the mass (this is a species of “solitary bee,” that is, they don’t nest communally, but a single female builds her own nest in the ground or in vegetation, and provisions the offspring with pollen rather than nectar.) All of these 42 bees were male. Nine of them tried to copulate with the mass, and when that happens a bunch of of beetle larvae climb onto the bee, temporarily debilitating it (see below).
The male bee falls to the ground and then grooms off most of the larvae—but he can’t remove the ones on his ventral service. (This is also true in the Drosophila I work with: when I dust them with fluorescent powder, they can groom all of it off except for some on their ventral side of the thorax.)
Now the male bee has a beetley venereal disease; in fact, every male has it: all the male bees that the authors sampled in one year carried beetle larvae.
The next problem is getting the larvae to females so they can be deposited in the nest. That’s essential because the beetle larvae eat the pollen that the female stores for her offspring. But transfer of larvae from males to females is easy: it happens during copulation. The author saw lots of females with beetles on their dorsal (top) surface, where they’d be transferred during mating, and witnessed at least one actual transfer during bee copulation.
The authors are careful to frame their hypothesis tentatively, which is of course that the beetle larvae have evolved a social behavior that helps them achieve the next stage of their life cycle: eating pollen in a bee’s nest. To that end, their behavior has evolved to make them aggregate and move as a unit. Genes that mandate this behavior would, of course, be advantageouos. This was (at least in 2000) the first known case of cooperative behavior in blister beetles, and the first known case of any cooperative behavior among individuals associated with mimicry. The authors also suggest that pheromone mimicry might be involved, since the bees appear to be interested in individual beetles before they form aggregations. Perhaps the beetle larvae (like some bee-fooling orchids) produce pheromones resembling those of female bees.
Note, too, that not all of the beetle larvae make it onto the bee (see comments below); many may die of dehydration. But note that they’re all brothers and sisters, and thus share many of their genes. You can thus see the communal behavior as the result of kin selection: you may die, but the genes for that behavior are present in your siblings who ride off on the male bee.
With that long introduction, showing once again the power of natural selection to create amazing forms of mimicry, I present the film that shows the whole ball of wax; the photography is remarkable:
And here’s a figure from the Nature paper showing a). an aggregation of beetle larvae on a twig (aggregations have a mean number of 549 larvae), b) A male bee with a bunch of beetle larvae on its ventral surface, and c) A female bee, presumably having mated with a beetle-laden male, showing the larvae on its dorsal surface.
Hafernik, J. and L. Saul-Gershenz. 2000. Beetle larvae cooperate to mimic bees. Nature 405:35.