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During its lifetime, a frog will snap up thousands of insects with its sticky, extendable tongue. But if it tries to eat an Epomis beetle, it’s more likely to become a meal than to get one. These Middle Eastern beetles include two species – Epomis circumscriptus and Epomis dejeani – that specialise at killing frogs, salamanders, and other amphibians.
Their larvae eat nothing else, and they have an almost 100 percent success rate. They lure their prey, encouraging them to approach and strike. When the sticky tongue lashes out, the larva dodges and latches onto its attacker with wicked double-hooked jaws. Hanging on, it eats its prey alive. The adult beetle has a more varied diet but it’s no less adept at hunting amphibians. It hops onto its victim’s back and delivers a surgical bite that paralyses the amphibian, giving the beetle time to eat at its leisure.
These grisly acts have been documented by Gil Wizen and Avital Gasith from Tel-Aviv University in Israel. Gasith discovered the beetle’s behaviour four years ago, when he found several larvae attached to frogs in the wild. Now, he and Wizen have worked out its strategy.
The larva lures amphibians by alternately waving its antennae and moving its jaws, moving faster and faster as its prey draws closer. The movements exploit the fact that amphibians run on simple hunting programmes: pay attention to moving objects; attack small ones; and avoid large ones. The small, waving larva certainly falls within the definition of ‘prey’; the amphibian approaches and attacks.
Its tongue is fast, taking just a tenth of a second to launch and extend. The larva is faster. Before it’s caught, it grabs the amphibian’s face and, after repositioning itself in a more suitable place, starts to eat. At first, it behaves like a parasite, sucking the bodily fluids of its prey. Then, those mandibles come into play and it starts to chew. Eventually, only bones remain.
The beetle always wins. In almost 400 face-offs, the amphibians only managed to get the larvae in their mouths seven times. Even then, they soon spat out the larvae, which quickly turned on them (first video below). One toad even managed to swallow a larva, which moved inside its stomach for two hours. For some reason, the toad eventually regurgitated its catch, and the larva, apparently unharmed, killed and ate the animal that had just eaten it (second video below).
The larva develops in three phases and it kills a new victim during each one. Once it eats its fill, it finds a hiding place, sloughs off its hard skin, expands its body and lures in a new amphibian to fuel the next stage of its growth.
As adults, the beetles often share the same moist shelters as amphibians during the day, only to prey upon them at night. In the wild, Wizen and Gasith found three beetles munching on toads from behind. In the lab, they saw that the beetle bites an amphibian in the back, hanging on like a rodeo jockey as its furiously jumping victim tries to dislodge it. The beetle makes an incision in its prey’s back with its jaws. Within a few minutes, it is paralysed. Within a few hours, it is nothing but a head and limbs.
The beetle’s cut doesn’t harm the amphibian’s spine. “I believe that the beetle damages the connecting muscles of the amphibian’s rear legs, thus preventing it from jumping away and escaping,” says Wizen. “This hypothesis still needs to be confirmed.”
Prey animals sometimes turn the tables on their predators, but that’s often because they’re bigger at some stage of their lives, because they suddenly outnumber their enemies. The Epomis beetles are unusual in three ways: they are much smaller than the amphibians they hunt; their role-reversal is compulsory since the larvae only eat amphibians; and they have evolved a behaviour that lures in their prey.
Clearly, frogs and salamanders haven’t learned that Epomis larvae are dangerous predators in their own right. That may be because the beetles are relatively rare, compared to the vast variety of ground beetles that amphibians regularly eat. Indeed, Wizen and Gasith found that the droppings of local frogs often contain the carcasses of other related beetles.
Only a few species of Epomis have turned the tables, transforming from prey into predators. Indeed, Wizen and Gasith think that the beetle’s behaviour first evolved as a form of defence. While other insects protect themselves with poison, camouflage, or aggressiveness, Epomis beetles rely on offence as the best defence.
Reference: Wizen & Gasith. 2011. An Unprecedented Role Reversal: Ground Beetle Larvae (Coleoptera: Carabidae) Lure Amphibians and Prey upon Them. PLoS ONE http://dx.doi.org/10.1371/journal.pone.0025161
Wizen & Gasith. 2011. Predation of amphibians by carabid beetles of the genus Epomis found in the central coastal plain of Israel. http://dx.doi.org/10.3897/zookeys.100.1526
Images and videos all courtesy of Gil Wizen