Beetle larva lures and kills frogs, while the adult hunts and paralyses them

By Ed Yong | September 21, 2011 5:00 pm
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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

Comments (26)

  1. Achintya

    This is one of the creepiest stories I have ever read. And then I watched the videos.

  2. lamanga2004

    Kind of amazing, but kind of yuck as well. There’s a related video showing the same beetle species eating a green tree frog. The frog is still very much alive as the beetle has already eaten strips of flesh from its back and legs. Then it moves onto the eyes…

    http://www.youtube.com/watch?v=rlMK7vRVa-I

  3. Scott sigler

    That is ten kinds of nasty and eight kinds of awesome.

  4. I will be merciless to all beetles found in my garden. I know everything is food but I like frogs.

  5. Robert S-R

    This is more terrifying to me than the ant-zombifying fungi, any number of parasitic wasps, or that fish parasite that replaces the fish’s tongue. Somehow, that beetle larva disturbs me on a whole new level.

  6. I actually thought that this would be about the larvae and adult beetle cooperating to hunt frogs, but this is even more crazy awesome.

  7. This actually astonishes me more than almost any insect I’ve learned about in recent years. I hope you don’t mind if I blog about this myself….this is too good not to spread.

  8. Andrew Forbes

    Tremendous! Another great natural history story, Ed.

  9. Georg

    With respect to the Yellow colour of the larva and being spat out when ingested, I’d look for
    some poison nevertheless.

  10. Frank

    Thanks for the nightmares. This is epic horror courtesy of the natural world – and I’m all freaked out now.

  11. Archwright

    Nature is nasty. That’s why we built walls. ^_^

  12. glen

    yikes! those are disgusting little creatures; one more reason to hate insects

  13. Cathy

    Yeah, normally I’m the sort of person who isn’t grossed out or creeped out by anything of this nature, but even this has give me the willies… probably because it’s one gross thing eating another gross thing alive. I’m going to have nightmares now, thanks.

  14. ferretgrrrl
  15. Yeah, this horrified me too. I think it’s the combination of moving very fast and then killing very slowly.

  16. MrO

    I think, Ed, that the revolving feeling of this has more to do with the fact that it contradicts the mental order we have of Nature: it is a disturbing combination of SMALL hunting and eating BIG (instead of the opposite) and “INFERIOR” form of life -according to our innate way of looking at it- winning over “SUPERIOR” form of life (instead of the opposite).

    Add to that the following concepts:
    - “UGLY AND CRUEL” killing “CUTE AND FAMILIAR”
    - an unseen, slow and painful way of killing
    - the infallibility of the larva (100% rate of succes!!)and the feeling that it is indestructible (alive and in perfect shape after 2 hours in the toad´s stomach!!)
    And THE RESULT IS HORROR

    (the larva, by the way, reminds me of “ALIEN, the 8th passenger”, the perfect killing machine: creepy, cruel, indestrutible, infallible, nightmarish…)

  17. Brian

    Being eaten alive slowly by something you thought was food…? That’s pretty horrifying.

  18. I wonder if the non-predatory relatives have very different behaviour, and don’t hang out in damp places with amphibians for example.

  19. george milton

    I suppose the beetle likes his food EXTRA FRESH

  20. Gil Wizen

    There are a lot of very nice ideas here. I will try to address some of the questions presented by commenters.

    Replying to Georg (#9): The Epomis larvae have color patterns that sometime seem aposematic (to warn off predators). When we checked the variation in color patterns we found out that larvae of the two Epomis species display color variation, and they can also have cryptic colors like brown and black. Link to publication:
    http://www.pensoft.net/journals/zookeys/article/1451/color-variability-and-body-size-of-larvae-of-two-epomis-species-coleoptera-carabidae-in-israel
    Moreover, we found no evidence that the larvae are poisonous. Their mandibles are not hollow, which means they cannot inject anything using while biting. Some ground beetle larvae have pre-oral digestion enzymes, so one of the next questions may be whether these enzymes cause the amphibians to be sluggish after larval attack. In ground beetles it is usually the adults who deploy all sort of chemical defenses.

    Replying to Arwen from the Chameleon’s Tongue (#18): The other ground beetle species related to Epomis (mostly species belonging to the genus Chlaenius) actually occur in the same moist habitats as Epomis, and very frequently we found the adults and larvae together with amphibians. We never recorded these species preying on amphibians. The Chlaenius larvae are “surface runners”, they actively search for prey and do not ambush like Epomis larvae.

    Replying to MrO (#16): I completely agree with your comment. This is why most people have a hard time to “digest” this story. But we have to remember that this interaction is natural occurs in the wild (at least on a seasonally basis), and that there is no “good” and “bad” in nature. Species are just doing their best to survive and to make sure they will produce offspring.

  21. PatrikD

    Thanks for jumping in, Gil!

    Can the frogs eat freshly killed larvae? That might be a good way to figure out if the frogs regurgitate the larvae because they’re poisonous, or because they’re actively irritating/damaging the frog gut.

  22. PatrikD

    As for level of creepiness, in comparison to the zombie fungus etc. – I think our brains are wired to feel empathy with other beings that are more like us. So mammals > other vertebrates > insects.

    Seeing the zombie ants is creepy, but how much empathy can you feel for an ant anyway? If we saw the same zombie fungus attack a frog, that would definitely be creepier, but it’s not as if the fungus itself strikes much fear – it doesn’t even move, except in time lapse. In the frog vs larvae case, not only do we feel empathy for the frog (it’s got a face, and big eyes), but it’s got an enemy for which we feel revulsion.

    It’s no accident that hollywood tends to model evil alien critters after insects – think the face hugger from Alien, the face of the Predator, etc.

  23. Gil Wizen

    PatrikD, wild amphibians will eat something only if it’s small enough and moves. Using killed larvae to test what you suggest might be difficult because the amphibians will not respond to them (this is what makes the larval luring so efficient).

    However, what you suggest is indeed an interesting question to test. There are different ways to address this question, they mainly involve presenting the amphibian with a manipulated (using Epomis larvae features) preferred insect prey.

  24. Christina

    That is really awesome! Creepy, yeah, but really really intriguing the way the insects reverse the normal pattern. I’d be really curious about how this evolved. Their ancestors would’ve been prey, presumably, so how did they switch from avoiding the frogs to preying on them? I would be very interested to see if anyone had any ideas about that!

    I also find it kind of darkly humorous, to imagine the frog thinking “Ooh, yummy prey” and then all of a sudden find itself the prey.

  25. Anton

    Being myself a biologist, I was surprised how much disturbed I was by those videos…

    On the scientific side, the thing that surprised me most is how the larva survived after beeing two hours in the amphibian stomach. I don’t know much about amphibian phisiology, but being animals that swallow their prey whole, they should possess fairly powerful gastric juices. Moreover, the larva didn’t seem to be affected by the lack of oxygen either.

    There is a philum of invertebrates, the Tardigrada, that can survive extreme conditions through cryptobiosis. I wonder if the Epomis larvae use a similar mechanism to survive. And if they can produce secretions that protect them from acidic conditions. I also noticed in the video that after being regurgitated the larva was immobile for some time. Maybe experiments in which the larvae are exposed to extreme conditions could make light on such questions.

  26. mastershake

    i dont think the succes rate of the beetle larva in the wild is 100% (its probably definitely between 90%-95%) but since some of the frogs were able to spit out the larva i would have to assume that in the wild they would spit out the larva then flee the scene (unless their instincts are so strong that they cant resist something that just attacked them), but a frog in a 1 ft diameter bucket cant escape the larva it just spit out so ultimately is attacked again by the larva

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