They scuttle through soil by the millions and have been successful since the time of the dinosaurs. Rove beetles, the most species-rich family in the animal kingdom, have long puzzled scientists with their evolutionary success. Now, a team of researchers may have discovered their secret weapon: a microscopic chemical factory hidden in their abdomens.

For years, researchers have wondered why beetles are so successful. There’s around 400,000 species of them that we know of, and there could well be another hundred thousand species that we haven’t discovered yet.

Beetles have several stunning innovations, most notably the elytra. This is a hardened, shield-like structure that protects the flight wings. The elytra enables beetles to thrive in ecosystems that are too dangerous for other insects. But this doesn’t explain the biggest beetle group of all: rove beetles (Staphylinidae).

These beetles have completely forsaken the elytra. They’re also predatory, contradicting the other popular idea behind beetle evolution (that they developed and diversified alongside flowering plants). For 200 million years, rove beetles have been doing their thing, covering any ecological niche you can imagine. So, what makes them so successful?

In a new study published in Cell, a group led by Caltech biologist Joe Parker has traced the success of rove beetles back over 100 million years to a surprising source — two cell types nestled in a gland at the tip of the beetle’s flexible abdomen.

These cells produce a noxious cocktail that repels predators, combining toxic benzoquinones with a liquid solvent. But it’s not just the gland’s firepower that matters. It’s its flexibility.

“The rove beetle tergal gland is this incredible, reprogrammable device for making new chemistries and evolving new interactions,” says Parker. “It enabled these beetles to achieve extreme forms of ecological specialization. Without the gland, there would have been no getting into the weird and wonderful niches that these beetles have found themselves.”

The gland’s chemistry has been repurposed in extraordinary ways. One rove beetle species secretes a sex pheromone used by mites, fooling its prey. Others live inside ant colonies, producing chemicals that pacify the ants, even turning the colony into an unwitting fortress.

Beetles are a bit like plants

The study, led by former postdoctoral scholar Sheila Kitchen (now at Texas A&M University), analyzed genomes across dozens of beetle species. The team found that the core genetic toolkit for the tergal gland has remained conserved for more than 100 million years. If something’s remained unchanged for that long, the odds are it’s doing something important.

“In piecing together the genomes, we were amazed by how similar the genetic architecture of the gland was across this massive group of beetles,” says Kitchen, an assistant professor at Texas A&M University. “It was when we started to look at specific gene families, we found hundreds of ancient genes that had found new functions within the gland, and a small but essential set of evolutionarily new genes.”

Beetles, it turns out, evolved a strategy eerily similar to plants. They bind toxins to sugar molecules to keep them inactive inside their own bodies, then release the active chemicals only after they’re safely secreted. They’re also able to release these toxins without actually poisoning themselves in the process.

“It’s pretty remarkable that chemically-defended beetles stitched together pretty much the same cellular mechanism as plants for not poisoning themselves with their own nasty chemicals,” says Parker.

That innovation, which evolved in the Early Cretaceous, may have kicked off a beetle explosion. Since then, the rove beetles have diversified into more than 66,000 known species.

“Once they hit upon this solution, it really took them places, evolutionarily-speaking,” says Parker.

Beetles are still evolving

Of course, evolution doesn’t stop. Not all beetles kept their glands. One group, so thoroughly embedded in the societies of army ants, appears to have lost the structure altogether. The species produces chemicals that pacify the normally aggressive ants. This allows it to live inside the colony, where it is protected; sometimes, it even eats some of the ants. Another species preys on mites and its gland is repurposed to produce mite sex pheromones.

“Apparently, once you have lived inside an army ant colony of millions of aggressive ants for long enough, you no longer need the gland,” says Kitchen. “We found that beetles that have managed to trick ants into accepting them into their societies had lost their glands during evolution.”

But for the vast majority of rove beetles, the tergal gland remains the centerpiece of their evolutionary theater. With just two cell types and a few key genes, they turned their bodies into living laboratories — able to adapt, repel, infiltrate, and thrive.

“The rove beetle tergal gland is this incredible, reprogrammable device for making new chemistries and evolving new interactions,” says Parker. “It enabled these beetles to achieve extreme forms of ecological specialization. Without the gland, there would have been no getting into the weird and wonderful niches that these beetles have found themselves.”

Sheila A. Kitchen et al, The genomic and cellular basis of biosynthetic innovation in rove beetles, Cell (2024). DOI: 10.1016/j.cell.2024.05.012