Prey eats the hunter as beetle larvae turn tables on toads

A beetle larva is attached to an amphibian host.Credit: Gil Wizen/AFTAU

September 2011. Ground beetles can immobilize and devour amphibian prey many times their size. Now Gil Wizen, a graduate student of Tel Aviv University's Department of Zoology, has discovered that they have an additional advantage - the larvae of these beetles, like their fully grown adult counterparts, have a unique method for luring and feeding off amphibians.

Wizen's research revealed that, like the sirens who lured Ulysses' sailors to their demise, larvae have a lethal method for attracting the attention of amphibians - tricking the toads into thinking they will be tasty prey.

In a dry country like Israel, amphibian species are already being threatened with extinction. Greater understanding of the larvae's habits and their impact on the amphibian population will have significant impact towards an accurate environmental risk assessment, says Wizen. 

Reversing the role of predator and prey
The project was set in motion when toad specimens were brought to Wizen's lab for observation. Discovering that some of the specimens had larvae attached to their bodies, the researchers noted over time that the larvae spent their entire life cycle feeding off the toads.

Adult beetles attack frogs
Adult beetles, Wizen says, ambush and then paralyze the amphibians by making a small incision into their back, perhaps severing the spinal cord or cutting a muscle so they cannot jump away. The beetles then consume and kill the amphibian. But the researchers wanted to know how this David-and-Goliath feat was accomplished, and collected more information on the larvae themselves and how they first attracted the amphibians' attention.

Larvae attach themselves when toads try to eat them
Amphibians hunt based on their prey's movement, Wizen explains. Larvae, immobile on the ground, attract the amphibian's attention by performing a sequence of movements, including opening their jaws and moving their antennae side to side, almost like a dance. When the amphibian tries to grab the larvae with its tongue, however, the larvae jumps and attaches itself to the amphibian with its jaws.

"It's really a predator-prey role reversal - the insect actually draws in its potential predator instead of avoiding it," says Wizen. "It's quite a unique phenomenon."

An unbeatable opponent
Although they are many times the larvae's size, the amphibians don't stand a chance, says Wizen. Researchers did observe some instances where the amphibian was quicker and managed to ingest the larvae, but success didn't last - in every case, the amphibian ended up regurgitating the larvae, which then attached itself to the amphibian's mouth.

Once the larvae has attached, the amphibian's diagnosis is grim. If the larvae are in the first stage of development, they will feed off the amphibian's body fluids like an exoparasite, and eventually, when they need to moult into its next developmental stage, they will fall off the amphibian's body, leaving a nasty scar.

But in the second or third stages of development, Wizen explains, the larvae begin to chew on the amphibians themselves, leaving behind nothing but bones.

The research was recently published in PLoS ONE.