Angle of Repose: Antlion

What’s up with this array of perfectly cone-shaped pits in the sandy soil along the foundation of the house? The pits are all exactly the same size with scribbly, doodly tracks in the sand between them. What we have here is an antlion larva, pronounced ant lion (Myrmeleon spp.). Its name is of old, showing the respect and awe this little creature evoked throughout human history. The lion part of its name speaks to the fact that it is a clever and ruthless hunter. The ant part reflects the fact that most of the creatures it captures are ants. It’s also known as a doodlebug because of the winding, spiraling tracks it leaves in the sand.

The antlion larva is an unlovely sight. It’s only about half an inch long, but it’s packed a lot of unlovely into that half-inch. It is dirt-colored, with a flattened, oval-shaped soft body, a plump abdomen and three pairs of walking legs. A slender mobile neck connects the thorax to the head. The head is large, square and flattened, and its main feature is a pair of oversized, Sickle-like jaws with several sharp hollow points. The jaws are designed to inject venom into its prey. The antlion also wears a coat of forward-pointing bristles that help it stay anchored in its sandy world.

Those cone-shaped pits in the sandy soil are traps set by the antlion to capture ants. They are cleverly designed to keep the hapless ant from climbing out once it has strayed in. To create the trap, the antlion walks backwards in a circle kicking sand up with its back legs and depositing it on its own head. The antlion then flicks its head just so to send the sand out of the pit. The antlion walks in a spiral, excavating the pit as it goes. When it’s done, and the pit is exactly right, the antlion buries itself under the point of the cone and waits, listening for the pitter-patter of little ant feet. Only its giant jaws stick out above the floor of the pit. When an ant steps over the rim of the pit, it starts a landslide that propels it downward toward those waiting jaws. If it tries to climb out, it only makes the landslide worse, and to add insult to injury, the antlion begins pelting the ant with sand. When the ant reaches the bottom of the pit, the antlion captures it in those awful jaws, and injects it with venom to subdue it. the next step is bathing it in digestive juices and then sucking up the ant’s liquified insides. When the antlion is done with its meal, it flicks the desiccated carcass of the ant out of the pit and buries itself again, waiting for its next victim.

Any fan of Star Trek II: The Wrath of Khan has seen one of these monsters being placed in Chekov’s ear. Eesh. Antlion larva photo from Wikimedia Commons by User:Wofl

The reason that the ant can’t climb out is that the antlion has dug the pit in such a way that the sand in the walls of the pit rests at the “angle of repose.” The “angle of repose” is the steepest slope where the sand will stay without collapsing into the pit. The sand is barely stable at this point, so when the ant steps on it, this little disturbance causes the fatal landslide.

When an antlion larva hatches, it begins setting its traps. Because it is dependent on hapless ants for food, it often has to go for long periods of time without eating. If winter comes, it will dig its way deeper into the soil and remain dormant until it warms up again. It may live as a larva for two or three years before pupating and becoming an adult.

Adult antlions don’t look like their childhood horrors. They look like drab, elongated dragonflies or damselflies. They are much larger than the larval stage, with clubbed antennae and intricately veined wings in browns and black. They are very poor flyers. As adults, they will mate, lay eggs and die within about 25 days.

I wonder what it would be like to have such seemingly different stages of life, going from fierce predator to an adult that can barely fly. Does the adult antlion remember its hunting days?

Feature Photo by Author. Alt text: A field of tiny, cone-shaped holes in the sand next to the house. Antlion traps.