Epomis beetles – insect response to amphibian tyranny

You can say that I am a little obsessed with Epomis beetles. Can you blame me? They are fascinating creatures. It suddenly dawned on me that since the launch of this blog I have not written a single word about the beetles. Unfortunately, there is a lot of misinformation and inaccuracies on the internet, and even in reputable magazines and books featuring Epomis.

It is one of the weirdest animal stories, one in which a small and seemingly harmless animal prevails against a much bigger animal. A unique case of predator-prey role reversal, where the would-be predator becomes the prey. Amphibians, such as frogs, typically prey on insects including ground beetles and their larvae. Among these beetles, one genus managed to stand out and deliver a proper counterattack to its predators. The Epomis larva has impressive double-hooked mandibles that look like they came right out of a horror movie. It waves them around along with its antennae until the movement attracts a hungry amphibian, which approaches quickly and tries to eat the larva. In a surprising turn of events, the larva is able to dodge the predator’s attack only to leap on the unsuspecting amphibian and sink its jaws into its flesh. It then continues to feed on the amphibian, sucking its body fluids like a leech at the initial stage, and eventually consuming it completely. Sounds like science fiction, I know. But it is real. Furthermore, these larvae feed exclusively on amphibians, and refuse to eat anything else. They are dependent on amphibian prey for completion of their development. This makes the predator-prey role reversal an obligatory one, which is very rare in the natural world.

First instar larva of Epomis circumscriptus showing its double-hooked mandibles.

First instar larva of Epomis circumscriptus showing its double-hooked mandibles.

I first learned about Epomis beetles in 2005, when I was working in the Natural History Collections at Tel Aviv University in Israel. They ended up being a great topic for my M.Sc thesis research, and I continue to study them to this day. The genus contains about 30 species distributed in the old world, with the African continent as the center of diversity. They inhabit the banks of rain-pools and temporary ponds, and synchronize their breeding season with amphibians’ metamorphosis into the terrestrial stage. Most of what we know about Epomis comes from studying three species only (in other words, there is more unknown than known). When the main paper from my thesis was published in late 2011, it became an instant hit in the media (see below). However, one main point of criticism was that the supplementary videos showed the interactions between Epomis and amphibians in a lab setting, which might have triggered an unnatural behavior from both. This is a valid point. We needed a controlled environment to test and prove beyond disbelief several hypotheses regarding the feeding habits of Epomis. Nevertheless, I spent the following years going back and recording the same interactions in the field.

Here is a larva of Epomis circumscriptus displaying luring behavior while waiting for a passing amphibian:

And this is the outcome of the above scenario:

 

To better understand what is happening during this swift encounter, here is a break down of this interaction to several simple steps. As you can tell by the above video, this sequence takes only a split second in real-time:

From enticement to desperation: European green toad (Pseudepidalea viridis) being lured to hunt and getting attacked by a larva of Epomis dejeani. View large!

From enticement to desperation: European green toad (Pseudepidalea viridis) being lured to hunt and getting attacked by a larva of Epomis dejeani. View large!

The larvae are terribly good at this. Even if they are caught by the amphibian’s tongue, they are still able to quickly use their mandibles to grab the amphibian from the inside, whether it is the throat or stomach, and start feeding.

Hard to believe, but this toad is being eaten.

Hard to believe, but this toad is being eaten.

Sometimes the amphibian accidentally steps on the Epomis larva. In this case, the larva will attach to the leg. First instar larva of Epomis dejeani feeding on a Lemon-yellow tree frog (Hyla savignyi).

Sometimes the amphibian accidentally steps on the Epomis larva. In this case, the larva will attach to the leg. First instar larva of Epomis dejeani feeding on a Lemon-yellow tree frog (Hyla savignyi).

While the larvae are specialized amphibian ambushers, the adult Epomis beetles are somewhat more generalist predators. They prey on other arthropods and will sometimes go for the occasional earthworm. But these feeding habits only last until they stumble upon an amphibian again. Then, a hidden memory back from the days they spent as larvae kicks in, and they set out to relive their glory days as amphibian slashers.

Epomis dejeani attacking a European green toad (Pseudepidalea viridis) while holding firmly to avoid falling off. Compare to the photo of the larva attached to the leg above.

Epomis dejeani attacking a European green toad (Pseudepidalea viridis) while holding firmly to avoid falling off. Compare to the photo of the larva attached to the leg above.

In a blink of an eye, the beetle sneaks up on the amphibian and pounces on it, holding firmly to avoid falling off. It then moves to the back, and like scissors uses its mandibles to make a horizontal incision, which disables the hind legs and ultimately prevents the amphibian from escaping. As if this was not gory enough, both adult beetles and larvae are particularly fond of eating the amphibian’s eyes. It is like a sick twist of revenge for the insects: after millions of years of suffering under the constant threat of predation by amphibians, they are able to fight back. Not only they hunt their potential predators and slowly eat them alive, but they also cripple them and peck their eyes out right from the start.

Remains of a partially eaten amphibian in the vicinity of temporary ponds are usually a good sign for adult Epomis activity in the area. Central Coastal Plain, Israel

Remains of a partially eaten amphibian in the vicinity of temporary ponds are usually a good sign for adult Epomis activity in the area. Central Coastal Plain, Israel

Epomis dejeani guarding a recently captured European green toad (Pseudepidalea viridis). The beetles can get very territorial over prey items.

Epomis dejeani guarding a recently captured European green toad (Pseudepidalea viridis). The beetles can get very territorial over prey items.

How did this phenomenon evolve? To be honest, we do not know exactly. But it is possible that somewhere in the evolutionary past, Epomis beetles used counterattack behavior, instead of escaping, as a defense against amphibians. Such behavior could have later evolved into exploiting amphibians as a source of food. The amphibians probably could have not evolved to recognize and avoid this behavior because the majority of insect prey they encounter poses no threat to them, as opposed to the relatively uncommon Epomis beetles. Another interesting point, is that both adults and larvae of Epomis lack any venom, yet the amphibian is quickly subdued and stops resisting after being caught, even while it is slowly being devoured alive.

One common reaction that I get in response to this study is that it was “cruel”, involving poor helpless amphibians that were sacrificed in the name of science. Some people even go further to suggest that I am a sadistic scientist somehow enjoying this. It could not be farther from the truth: This is a natural phenomenon and Epomis beetles must kill and consume amphibians in order to exist. Nature is cruel. We tend to think of amphibians as cute and helpless animals, but from the insects’ perspective they are actually cold-blooded killers (pun intended), gulping every small creature in their path. Moreover, the reality of this study is even harsher: the amphibians would have still died even without me using them as food for Epomis, because the puddles they were found in as tadpoles were quickly drying out. As for myself, I cannot begin to describe the emotional stress I suffered during this research, just so I could bring Epomis’ fascinating biology to the spotlight. I love amphibians, and it was disheartening for me to watch them die so many times. Throughout the study I kept telling myself: “I am going to hell for this, no doubt about it”.

In the past few years I have been following the response to the story of Epomis beetles. More sightings of the beetles are being reported from around the world. There are some excellent blog posts (1,2,3,4, and do not miss Bogleech!), news reports (1,2,3,4,5), videos and TV segments, radio interviews and podcasts, and even Wikipedia pages. Epomis has found its way into artwork. There is a metal band named after the beetles. It is very possible that this is the discovery I will go down in history for, and that is fine by me. Hollywood, I am waiting by the phone for your call. To end this post on a positive note, here is a fitting limerick that I love, written by the talented Celia Warren:

Of the genus Epomis, folk say,
Their larvae at first seem like prey,
But they’ll bite a frog’s throat,
Leave it paralyzed, note!
Then they’ll eat it without more delay.

20 thoughts on “Epomis beetles – insect response to amphibian tyranny

  1. Amazing, this may be my favorite post on this blog! I find this genus endlessly interesting, both in behavior and in looks. I mean come on, double hooked mandibles, that is like something out of a nightmare! 😀

  2. Thank you for such an interesting article and great pictures! From now on whenever I see any beetle resembling one of these I will be sure to kill it mercilessly. :3

    • And why would you do that? Just because this beetle is a predator doing exactly what it is supposed to do?
      Would you kill a lion or a fox too?

  3. I’m kind of curious when I read that bullfrogs are an invasive in Israel now. I know that they are far larger than the frogs normally preyed on by epomis. Have you observed any interaction between the two and if so what were the results?

    • I wonder where did you read it? As far as I know there are no bullfrogs in the wild in Israel. At this point there is no evidence of Epomis attacking or feeding on the big invasive amphibian species (bullfrog and cane toad). However, consider that Epomis beetles respond to amphibians regardless of their species, so it is possible. Size does not really matter in this case, there are records of Epomis adults and larvae successfully feeding on native adult toads and frogs, which are quite large.

      • I saw it here which lists Israel as one of the countries and have been invaded by rana catesbeiana

        http://www.cabi.org/isc/datasheet/66618

        When you say they feed on adult toads and frogs are they actually able to kill them and do they get as large as cane toads and bullfrogs? (Curious because I saw one of your papers which listed masses of I think up to 5 grams where as a typical bullfrog is between 50-500 grams.) Also curious since bullfrogs are known for swallowing and not spitting out some normally fearsome insects. (Such as hornets, wasps, and giant water bugs.) On larger frogs do they simply parasitize them?

        • Thank you for the link, I was not aware of that. As of now, I could not find any report in Hebrew about Rana catesbeiana occurrence in Israel. Maybe it has already been eradicated.

          In the case of Epomis, there is some new information that is not mentioned the papers (still unpublished), so I cannot give too many details at this point. We reported about amphibian metamorphs (juveniles), which is the most common prey type for Epomis during the time of its activity. Occasionally, the larvae and adults attack and feed on adult amphibians which are larger. Whether they are able to kill an adult amphibian depends on several factors, such as the size and weight of the amphibian, and the severity of damage inflicted during the attack. There is not enough information to determine the outcome of a bullfrog/cane toad-Epomis interaction. However, keep in mind that the adult Epomis beetles have their own strategy for taking down a large-sized amphibian, and they are very successful.

          • Very informative. Oh one final question. In the course of your research did you ever investigate if on top of everything else are these beetles toxic to amphibians? Given everything else they’ve got going for them I wouldn’t be surprised if they were. (Come to think of it the easiest way I could think to test this would be to see if amphibians could eat either adults or larva that either were decapitated or simply had their mandibles cut off.)

          • Epomis species have chemical defense, but they are not more toxic than their closest relatives, Chlaenius species, which are readily eaten by amphibians.

  4. Extremely fascinating! I have studied certain predatory beetle larva, seems the ground beetle larvae have either hollow mandibles, or a groove down the inside of each and that the larvae inject saliva into their prey which helps digest the insides enabling the larvae to suck out fluids. Here with the Epomis larvae, I see an extra set of hooks, probably to help the larva hook it’s mandibles to the frog skin or tongue, but the main hooks are those with the channels for sucking fluid and injecting digestive/anti-coagulant fluids into the flesh? Are the later instars able to actually chew off pieces of the toad’s flesh?
    While living in Southern Japan, I observed large Carabid larvae eating small toads/frogs in a similar manner, but these larvae were much larger. I also found beautiful large Carabid adult beetles doing the same thing, and they chewed their prey apart. I also observed these same predators killing and eating land-snails. In the case of snails attacked by the larvae, when I tipped up the shell, most of the left-over snail body was liquid. With those attacked by adults, the chewed-up flesh remained…

    • Thank you!
      According to SEM photographs we took during the study, larvae of Epomis do not have hollow mandibles. No holes, channels, or anything along these lines. The hooks are there to serve the function of… well, hooks. To prevent the amphibian from removing the larva. In other words, the larvae do not inject saliva into the amphibian body, but rather secrete it on the wound and later consume the dissolving tissue. The third-instar larvae indeed do more chewing while feeding.


Leave a Reply

Your email address will not be published. Required fields are marked *

*