We know frogs as predators that gobble up insects – we see them sit very still and grab unsuspecting prey with a quick flick of the tongue. But in nature, sometimes, this predator–prey relationship is turned on its head: the frog falls prey to the same insect it set out to hunt.

Meet the Epomis ground beetle. Found in parts of Asia, Africa and Europe, Epomis beetles go through the typical life stages of egg, larva, pupa and adult. Though adult beetles are generalists that eat pretty much anything, their larvae have specific dietary needs: they feed only on amphibians.

So, it is no coincidence that the beetles live and breed in the same habitat as amphibians. They synchronise their breeding season with the amphibians’ in a way that their eggs hatch when lots of young amphibians are around.

That’s because the larvae will readily attack any newly minted amphibian, including a frog, toad, salamander or newt, even one that is 5 to 20 times their weight. The larvae don’t need to actively hunt their amphibian prey; they make their prey come to them.

Here’s what a three- to nine-day-old larva does: it stays put on the ground and repeatedly sways its antennae and jaws. It waves and waits. This waving is inviting to a young amphibian that might be looking for insect food. As the amphibian, for its part, inches closer in the hopes of a tasty treat, the larva starts to wave more vigorously.

Basically, what it does is mimic insect-like movements but the larva itself does not move, said Gil Wizen, an entomologist at the University of Toronto in Canada, who described the luring behaviour of beetle larvae in 2011. “That attracts amphibians because amphibians hunt based on the prey’s movement – they just lock onto the prey and then fire their sticky tongue to catch the insect,” he explained.

But before the amphibian’s tongue can grab it, the beetle larva quickly hooks onto its body. The larva is fast and takes advantage of the already-close amphibian to grab it where it can, he said. “It’s usually the throat area but sometimes it’s the leg or the side of the body.”

The larva, now lodged deep into the amphibian’s flesh, hangs from the amphibian’s body and sucks its body fluids – like a parasite living and feeding on its host.

It isn’t surprising if you’ve never heard of this before. It’s a rare occurrence and, in India, has only been reported twice so far. The first instance, of an unidentified beetle larva feeding on an adult Ferguson’s Toad, was published back in 2011. The toad with the larva attached to it seemed weak and struggled to hop when scientists found it on the lawn of a museum in Mysore, Karnataka.

In September 2017, a different group of scientists published the second account – this time from Amboli, Maharashtra. Here, the beetle larvae were found feeding on the Amboli Bush Frog (Pseudophilautus amboli), a critically endangered species. Wildlife researchers Anish Pardeshi and Kaka Bhise of the Malabar Nature Conservation Club were surveying amphibians of the Amboli forest in late 2016 when they noticed something a bit odd about a few frogs.

Larvae of the Epomis ground beetle feeding on a frog
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The larvae of the Epomis ground beetle don’t need to actively hunt their amphibian prey; they make their prey come to them. Photograph: Kaka Bhise

Five bush frogs had an insect larva dangling from their flesh; these were perched on leaves of plants close to the ground. Having never come across the larva before, Pardeshi and Bhise turned to experts for help. They posted its photograph on the Entomology group on Facebook where members help each other identify species.

A manager of the group, who was familiar with Wizen’s work on frog-eating larvae, saw the post and tagged him in the comments. Wizen identified the larva as belonging to the Epomis ground beetle and thus ended up collaborating with Pardeshi and Bhise. The trio summarised the findings in the journal Herpetological Review.

Interestingly, the five larvae-infected individuals that Pardeshi and Bhise had come across were all juvenile frogs – none were fully developed adults. Young frogs, Wizen told me, require a lot of food to grow and are more likely to approach the waving insect larvae. “That’s how they get infected.” The juveniles also stay close to the ground, said Pardeshi, where they are more at risk because the beetle larvae themselves are ground-dwellers.

Adult bush frogs, which live higher up in the trees, are usually well out of the reach of larvae. “It would take a very special case for the two of them to meet,” said Wizen. So while the adults are relatively safe from these beetle larvae, for juveniles attacked by the larvae, the chances of death are high. Most of the amphibians that do get infected die, said Wizen.

And it is a slow death for these unlucky ones. A young beetle larva, in its first phase of development (first instar), takes about four days to suck its frog meal dry. After this, it lets go of the amphibian and starts moulting. Then it sets off in search of another amphibian, stepping up its game.

Bigger (second and third instar) larvae also create antennal movements to attract an amphibian but they also move much more, said Wizen. “They can actually take a few steps [towards it] instead of just waiting for it to approach.”

These larvae, at about 10 to 16 days of age, have much stronger jaws and make the switch from parasitic to predatory behaviour. They can chew the flesh and eventually eat the whole amphibian, leaving nothing but bones, explained Wizen. A few hours are now enough for the larvae to bring an end to their prey.

By the time a beetle larva turns into an inactive, non-feeding pupa, it will have consumed up to four amphibians. If this turns out to be true for the Epomis species found in India, it could spell trouble for the Amboli Bush Frog, already a critically endangered species found only in the Western Ghats.

Amboli Bush Frog affected by larva of an Epomis beetle
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It is a slow death for these unlucky juvenile frogs. A young beetle larva, in its first phase of development takes about four days to suck its frog meal dry. Photograph: Kaka Bhise

But Wizen and Pardeshi caution against sounding the alarm yet. Both said we need more research to assess the percentage of the frog population affected by the larvae. They are a natural predator, Pardeshi pointed out. It is not a case of unnatural predation from an invasive species or disease, added Wizen. And nature has its own balance.

The larvae strictly depend on the amphibians for their own survival, so they cannot go on and attack the whole population because that would be their doom, said Wizen. And it’s not like they’re successful every time they hunt, either. Sometimes, the larvae miss – and don’t catch the amphibian, he said. “If the larva doesn’t find amphibian food within six days it dies of starvation.”

However, among a lot of things we don’t know is whether the Amboli Bush Frogs get predated upon by beetle larvae alone or their adult forms as well. Though they are generalist predators, adult beetles do have a taste for amphibians. Adult beetles, Wizen said, can kill an amphibian if they bump into one. If one is close enough, they will grab it, bite to paralyse it and then feast on it. In some countries, such as Japan and Israel, researchers have observed adult beetles preying on frogs and toads.

“Generally, amphibians feed on insects – they feed on hundreds and thousands of different insects throughout their lifetime. What we have here is a very unique case of role reversal,” said Wizen. It is possible that this reversal of roles evolved as a means for the beetles to fight back their amphibian predators. But at this point, he said, it’s only a theory.