Ant lions hunt despite sealed lips

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Ant lions are ferocious predators, but some of them don’t have a mouth.

At least not in the usual sense. Over evolutionary time, the slit at the front of the mouth cavity has sealed shut in the armored larvae of ant lions that hunt in sand.

Only young sand-dwelling ant lions are mouthless. As adult insects, the 2,000 or so named species in the ant lion family, the Myrmeleontidae, have mouths and often a touch of the nature-calendar prettiness of damselflies, with long, lacy wings. But “they’re not the killing machines that the juveniles are,” says biologist Sandra Binning of the University of Neuchâtel in Switzerland.

Instead of mouths, the larvae rely on two long, toothed hooks at the front of the head. The youngsters dig pits in sand and bury themselves up to the hooks, ready to grab and stab ants or other insects that tumble in.

Each hook has a covered groove on the underside. When the hook pierces prey, the ant lion injects venom and digestive enzymes via the grooves. Then the same grooves deliver paired streams of liquefied prey back directly into the mouth cavity, says Mervyn Mansell of the University of Pretoria in South Africa. Feeding by grooves is much like “if you drank a cool drink through two straws,” he says.

At least at the front end. The gut of a larval ant lion dead-ends partway through its body. The larva can pee, but solid waste builds up until adulthood when the gut grows an exit.

With no mouth opening and no way to poop, sand-trapper ant lions minimize their intake of anything indigestible. After slurping ant juice, they fling the carcass out of the pit to lie around the rim. The resulting lawn art might scare other ants away, Binning and her colleagues at the Australian National University in Canberra thought, so they tested Myrmeleon acerant lions that they had scooped up with big spoons from under rental trailers near a beach. “The tourists thought we were nuts,” she says.

Fresh carcasses lying around reduced a trap’s effectiveness, though apparently not as terrifying warnings, the researchers report in an upcoming issue of Ethology. Living ants seemed to be drawn to dead ones. Stopping to investigate reduced the chances an ant would blunder over the trap’s edge.