The predator-prey relationship between frogs and beetles seems like it would be pretty obvious doesn't it? Frog spots beetle, frog stealthily approaches beetle, frog eats beetle. Done, done and done.


But it turns out that the relationship between amphibians (like frogs and toads) and the larvae of ground beetles is one of nature's rare examples of role-reversal within a predator-prey interaction. That's right — in the case of amphibians and ground beetle larvae, the hunted larvae actually become the hunters; while the amphibians become the prey.

Scientists have known for some time that the larvae of ground beetles from the genus Epomis feed exclusively on amphibians, but how the balances of a predator-prey encounter between a frog and a larva could possibly tip in favor of the diminutive larva had long been a mystery.


Now, Tel-Aviv University's Gil Wizen and Avital Gasith have discovered how the larva manages to accomplish this impressive feat, in what they describe as "an unprecedented role reversal." The researchers explain in their paper, published in today's issue of PLoS ONE:

The Epomis larva combines a sit-and-wait strategy with unique movements of its antennae and mandibles to draw the attention of the amphibian to the presence of a potential prey. The intensity of this enticement increases with decreasing distance between the larva and the amphibian. When the amphibian attacks, the larva almost always manages to avoid the predator's protracted tongue, exploiting the opportunity to attach itself to the amphibian's body and initiate feeding. Our findings suggest that the trophic interaction between Epomis larvae and amphibians is one of the only natural cases of obligatory predator-prey role reversal.

In other words, the larvae actively lure frogs and toads to their fate by tricking them into thinking that they're the ones doing the hunting, when the real hunters are, in fact, the larvae.


When an amphibian strikes, the larva evades the attack with what the researchers describe as a swift head movement in the direction of the pouncing amphibian.

The larva uses its formidable-looking mandibles (shown above) to latch onto the nearest part of the amphibian's body before repositioning itself and begining its feast. The larva starts its meal by sucking on the frog's bodily fluids, but eventually shifts to full-blown mastication, gnawing away purposefully at the amphibian's flesh.


Pictured here are the bones that remain once the larva has finished with its prey. What's most impressive about the whole process is just how many of the amphibians meet this gruesome end; in Wizen and Gaseth's experiments, they pitted larvae against frogs and toads over 400 times, and not once did the amphibian manage to survive the encounter.


The few amphibians that did manage to get their mouths around a larva — or even swallow one — either spit the larva back out, or wound up regurgitating it later.

But even in these rare cases, the amphibians eventually succumbed to the same fate as all the others.


"Such role reversal," write the authors, "is exceptional in the animal world, extending our perspective of co-evolution in the arms race between predator and prey."

Via PLoS ONE (No Subscription Required)

Images via PLoS ONE

Videos by Gil Wizen via Ed Yong