How Antlion Larvae Eat Without a Mouth

Gilles San Martin

The larvae of antlions are sometimes referred to as "doodlebugs" because of the meandering trails some species leave in the sand as they wander around looking for a place to settle in and find some food. Don't let the cute nickname or the unintentional artwork fool you, though. The young bugs look more like something you'd find in an H.P. Lovecraft story than on Zooborns. 

While adult antlions can be beautiful, with a slender dragonfly-like body and delicate wings that are crisscrossed by veins, the larvae are grotesque looking, even by the standards of insects. Their bodies are bulbous and hairy, and their flat heads sit on elongated necks and feature a pair of sickle-shaped mandibles. And those cute little doodles they leave on the ground? They end when the larva finds the perfect place to build a death trap.

The larva constructs this trap by walking backwards in circles while flicking sand and small rocks away. As the bug goes round and round, the circles get smaller and the trap gets deeper, until there's a conical pit. The trap is now set and larva buries itself at the bottom, with only its mandibles protruding from the sand. As ants and other insects scurry by the pit, they tumble in and slide down the steep walls. They can try and walk back out, but find little footing in the loose sand. The waiting antlion can speed up the prey's demise by flinging sand at it, causing a little landslide that brings the ant to the bottom of the trap. Then, the antlion eats. 

Amazingly, it does this without a mouth that works like most animals'. Instead of a mouth that opens and shuts, the larva has only a fixed, shallow slit that's of little use for biting or chewing solid food. It has to liquefy its meals, and it does this with those curved mandibles. The antlion grabs and stabs its prey with the hooks, injecting it with digestive enzymes that dissolve its soft tissues, and then sucks the nutritious goo back out.

Since it's got no use for the corpse that's left, the antlion simply tosses it up and away to lie on the rim of the trap. If an antlion eats well, this trash pile can get pretty big, and scientists in Australia wondered what that means for the bugs. Ants can pick up on the chemical odor signals that waft from other ants and their corpses, and so, on one hand, the carcasses might attract other curious ants that wander over to investigate and fall into the trap themselves. On the other hand, they might provide alarm signals for other ants that tell them to avoid the area. 

The researchers built several artificial antlion pits in their lab. For some of the pits, they littered the rims with either fresh ant carcasses or older ones. Other pits had ant-sized pieces of bark placed on the edges instead of dead ants, and some were kept clear. Then they dropped ants into the containers to see how they'd behave around the different traps.

There weren't any differences in how quickly or how often the ants approached the various pits. And while they did spend more time near the pits that had debris and examined the carcasses and bark, they fell into these traps less often. The type of object laying near the pit—whether it was a fresh corpse, or an old one, or a piece of bark—didn't seem to matter, and any kind of litter made it less likely that the ants would fall in. This suggests, the researchers say, that dead ants around the pits attract live ants, but also make them less effective as traps. 

The researchers think that this is because the ants will slow down or stop to investigate small objects around the pit, which prevents them from scurrying headlong into the trap. While a messy rim can deprive the pit's owner of a meal, it can be a boon for its neighbors. Antlions tend to dig their pits in clusters, and an ant that's attracted to the area by the carcasses at one pit has a good chance of falling into another one nearby that's kept a little cleaner.