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Bombardier beetles – explosions of smoke in your face

I was very positively surprised by the response to my previous blogpost about Epomis. In fact, it now seems that this post is the most popular one on the blog, even more than the ones about the botfly and my NZ accident. How do I top it? Only time will tell. In the meantime, I wanted to mention some of the other ground beetles (family Carabidae) that share the habitat with Epomis. You see, when you start flipping stones and pieces of wood scattered around rain-pools you encounter many carabids. But one group really stands out in appearance, and, as much as it is hard to believe, in sound: the bombardier beetles.

Bombadier Beetle (Brachinus crepitans), one of the cutest species of ground beetles. Golan Heights, Israel

Bombadier Beetle (Brachinus crepitans), one of the cutest species of ground beetles. Golan Heights, Israel

An aggregation of several beetle species found under a rock. Bombardier beetles (Brachinus alexandri) can be seen on the right. Also appearing in this photo: Chlaenius aeneocephalus (Carabidae, metallic colors), and Cossyphus rugulosus (Tenebrionidae) - beautiful beetles camouflaged as seeds! Central Coastal Plain, Israel

An aggregation of several beetle species found under a rock. Bombardier beetles (Brachinus alexandri) can be seen on the right. Also appearing in this photo: Chlaenius aeneocephalus (Carabidae, metallic colors), and Cossyphus rugulosus (Tenebrionidae) – beautiful beetles camouflaged as seeds! Central Coastal Plain, Israel

Bombardier beetles is a large group comprised of several Carabidae tribes. Here I refer mainly to species of the genus Brachinus. These are small to medium sized beetles, usually with striking aposematic coloration: the body and limbs are bright orange, while the elytra (wing covers) are usually dark green or brown, sometimes with a metallic sheen. These colors serve as a reminder for potential enemies that these beetles can deploy a powerful weapon: an explosion of hot chemicals, which can be aimed at almost any direction.

Two common species of bombardier beetles from Israel: left - Brachinus alexandri; right - Brachinus berytensis

Two common species of bombardier beetles from Israel: left – Brachinus alexandri; right – Brachinus berytensis

Much has been written about the mechanics and evolution of the beetles’ chemical defense. In short, when provoked the beetle releases two chemicals, hydroquinone and hydrogen peroxide, into a chamber in its abdomen. This mixture, when comes in contact with a catalyst, turns highly combustible due to the oxidation of hydroquinone and the breakdown of hydrogen peroxide to oxygen and water. The chemical reaction starts inside the chamber with temperatures reaching 100°C, and the high-pressure buildup causes the explosion. Then all the beetle has to do is to aim its “nozzle” and fire! The result is a smoke cloud of chemicals at extremely high temperatures. It can momentarily paralyze or even kill arthropod enemies, such as ants and spiders. To us humans (=entomologists who collect the beetles with bare hands) the damage it causes is not so severe, usually nothing but a small stain of burnt tissue, but the effect is coupled with a startling popping sound, and that might be enough for the beetle to escape from a large predator. This complex defense mechanism was used by creationists as an example for intelligent design in debates against evolution. However, it can be easily demonstrated that by gradually increasing the concentration of hydrogen peroxide this defense could evolve in incremental steps without risking the beetles’ existence. If you are still confused, I highly recommend watching Richard Dawkins explaining it here.

Damage to skin caused by bombardier beetle (Brachinus berytensis) chemical defense

Damage to skin caused by bombardier beetle’s (Brachinus berytensis) chemical defense. Not much.

I feel that I must stop here for a brief public service announcement: There are several videos showing the beetle’s defense (you can google them), almost all of them depict the beetle being held in place with either glue or a pair of tweezers. I would like to argue that unless this is being done for research purposes, these actions border on animal cruelty. Sure, it is strange to hear such a statement coming from someone who fed live amphibians to beetles. Still, I want to stress that in the case of the bombardier beetles this is highly unnecessary. The beetles will still put up the same “show” if poked or gently lifted, without causing them much stress and damage, as can be seen from this short video I took almost a decade ago (I mean it, this is a really old video, so please do not judge the quality):

The species shown in the video is Brachinus bayardi, one of the largest species found in Israel:

Bombardier beetle (Brachinus bayardi), Central Coastal Plain, Israel. These beetles are strictly nocturnal, and can be found running on muddy banks of rain-pools in search of prey.

Bombardier beetle (Brachinus bayardi), Central Coastal Plain, Israel. These beetles are strictly nocturnal, and can be found running on muddy banks of rain-pools in search of prey.

While the chemical defense of the bombardier beetle alone is interesting enough, there is another aspect in their life history that is fascinating. In most species, the adult bombardier beetles are predators of small, soft-bodied invertebrates, but as larvae they feed solely on pupae of other beetles found in the same humid habitat, usually diving beetles (family Dytiscidae) and water scavenger beetles (family Hydrophilidae). This makes them parasitoid insects – their larvae are completely dependent on another insect for completion of their development, usually with fatal consequences to the host. While most parasitoid insects are wasps and flies, in beetles this way of life is relatively uncommon, with only a handful of beetle families exhibiting a parasitoid life history. Despite searching for years, I have yet to find larvae of bombardier beetles, and my attempts to obtain larvae from captive adults has failed so far. I hope this will change one day.

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.

The incredible tadpole shrimps

One of the creatures I wanted to find during my visits to Israel is a big crustacean, found only in temporary ponds during a specific time of the year. In previous years I was unlucky to find it – I visited in the late spring and early summer, and most of the rain-pools were already gone or in the process of drying out.

But this year I planned my trip way ahead, making sure to save some time for searching these animals.

Tadpole shrimps belong to the small order Notostraca, which contains a single family, Triopsidae, with only two genera: Triops and Lepidurus. These animals are considered living fossils, having not changed significantly in appearance since the Triassic period, about 200 million years ago. They also bear a strong resemblance to horseshoe crabs, characterized by a broad, shield-like carapace, which conceals the head and bears three eyes, a pair of compound eyes and a nauplius eye between them. The abdomen is long, and ends in two caudal filaments. But in my opinion, the real wonder about tadpole shrimps is their large size. Some species can reach a whopping length of 10cm, larger than most insects and amphibian tadpoles co-habiting in the same pond. And indeed, many people I met while surveying rain-pools could not believe that this was a crustacean until I pulled one out of the water.

Temporary ponds like this one are home to various aquatic invertebrates such as the tadpole shrimps. Unfortunately, these habitat are threatened with destruction in Israel. Here a low wooden fence is the only physical barrier between this small nature reserve and a sand quarry found behind the pond.

Temporary ponds like this one are home to various aquatic invertebrates such as the tadpole shrimps. Unfortunately, these habitat are threatened with destruction in Israel. Here a low wooden fence is the only physical barrier between this small nature reserve and a sand quarry found behind the pond.

 

Portrait of a Tadpole shrimp (Triops cancriformis), showing two compound eyes and a middle nauplius (larval) eye.

Portrait of a Tadpole shrimp (Triops cancriformis), showing two compound eyes and a middle nauplius (larval) eye.


There are two species of tadpole shrimps in Israel. The bigger one, Lepidurus apus, is relatively common and can be found in many ponds along the coastal plain from winter to the early spring. It is easy to recognize – not only is it big and brightly colored with red and olive-green, but it also sports a wide transparent scale at the end of its body, between the two caudal filaments.

Tadpole shrimp (Lepidurus apus). The bright red color indicates presence of hemoglobin, an adaptation for life in habitats poor with oxygen.

Tadpole shrimp (Lepidurus apus). The bright red color indicates presence of hemoglobin, an adaptation for life in habitats poor with oxygen.


The second, smaller species, Triops cancriformis, is much more scarce, and in fact is known only from a handful of ponds. This species is critically endangered in Israel due to habitat destruction, however it was recently recorded from a new location, a water reservoir in the Arava desert, the southernmost point in its distribution to date. A little less colorful than its close relative, it is mottled with green and grey splotches that assist in blending in with its surroundings.

Tadpole shrimp (Triops cancriformis) digging in the sediment. The green and grey splotches make excellent camouflage for concealing it from predators striking from above, such as water birds.

Tadpole shrimp (Triops cancriformis) digging in the sediment. The green and grey splotches make excellent camouflage for concealing it from predators striking from above, such as water birds.

 

A side view of a tadpole shrimp (Triops cancriformis) revealing eleven pairs of legs. The first pair is long and modified to function as a sensory organ.

A side view of a tadpole shrimp (Triops cancriformis) revealing eleven pairs of legs. The first pair is long and modified to function as a sensory organ.


Both species are omnivores, living on the bottom of the pond while relentlessly digging in the sediment searching for food. Occasionally they are seen swimming close to the water surface, especially in shallow parts of the pond, where they are usually mistaken for amphibian tadpoles, hence their common name.
But the most fascinating fact about the life cycle of tadpole shrimps is that similarly to other aquatic invertebrates they too must face the inevitable faith of the temporary pond: drying out. For them, it is a race against time; they must grow fast, mate and lay their eggs before the pond disappears completely, killing every one of them in the process. Once the pond fills up in the winter, the tadpole shrimps hatch from their eggs, and grow at an impressive pace, reaching their adult stage in just a few weeks. Then, they reproduce, but the context of reproduction depends on the population. In some ponds tadpole shrimps reproduce sexually, but in many populations the males are absent, and the females reproduce asexually, in a process called parthenogenesis. They release unique, long-lasting eggs that can stay dormant for many years, buried in the dry soil. This way, they can “skip” several years of drought, during which ponds have tendency of evaporating too fast. When there is sufficient precipitation and the pond fills again with rainwater, some of these eggs hatch (others stay in further dormancy for the following years), and the cycle starts again.
I was very happy to find these lively creatures in my last visit to Israel and could not resist taking some photos of them for Meet Your Neighbours project.

Tadpole shrimp (Triops cancriformis) from central Israel. The shield-like carapace gives it the appearance of a small horseshoe crab.

Tadpole shrimp (Triops cancriformis) from central Israel. The shield-like carapace gives it the appearance of a small horseshoe crab.