Have you ever noticed a line of funnels dotting the sand at the base of your house, just inside the drip-line of your gutter or roof? Funnels about an inch across, and so regular that they could not possibly be due to raindrop drips?
These are the homes — and traps — of a fearsome insect predator (have no fear yourself) called an ant-lion.
Recently, I led an insect foray at the new home of the Hitchcock Center for the Environment, and we discovered a group of these insects nesting just below the center’s front roof, just to the left of where everyone enters this new building. People aged 6 to 60 got down on their hands and knees to admire these interesting formations, some having recognized them from their childhoods, others learning about them for the first time. Much giggling ensued.
Welcome to the world of ant-lions, called by this name because they prey on ants and other insects that fall into their traps. They are larvae, the immature forms of insects in the order “Neuroptera” (or “nerve wings”), the order named for their wings that are rich in beautiful networks of nerves. You may occasionally spy some green, aptly named lacewing insects — their close relatives — on your travels; these are delicate examples of insects in this group. Even though they are scary in their early stages, ant-lions are lovely as winged adults, as they briefly live, fly and mate before they end up as food in bird beaks and in mouse mouths.
Before they turn into “swans,” though, ant-lions start out as “ugly ducklings.” As larvae the size of a human fingernail, they are mottled brown or gray, covered with fuzz and spiky-looking black bristles. Their coloration is superb camouflage for their sandy habitats. Most formidable are their out-sized mandibles (grasping jaws), which jut out from their square-ish heads like lobster claws.
Ant-lions can remain in the larval stage for as long as three years, ambushing prey until they enter the next phase of their lives. To prepare a trap for its prey, the ant-lion first has to choose the perfect location, and it is picky. It requires dry sand grains that are small and uniform in texture.
Experiments by Nicholas Gotelli at the University of Vermont have shown that ant-lions prefer areas that are protected from rain but that still receive some warming sunlight; wetting the sand causes the ant-lions to look elsewhere for real estate because rain crusts the sand and makes it difficult to dig.
Ant-lions reach the northern edge of their range in Vermont, and need all the warmth they can get. This choosiness may explain why ant-lions congregate in linear groups just beneath a south-facing roof drip-line.
Next, the ant-lion digs a conical hole, using its mandibles to toss sand out of its growing pit. It assiduously jettisons small pebbles out of the funnel, and gradually backs down into the sand, concealing itself to await prey at the bottom of its pit. The soft sand lies at the angle of repose — the steepest angle at which a sloping surface formed of a loose material is stable. If you’re a golfer, think about how hard it is to lob a golf ball out of a sand trap.
Now, a busy, foraging ant comes into the picture. She — all ant workers are female — stumbles and falls into the funnel. She tries repeatedly to surmount the walls of the pit, but it’s a Sisyphean task — each of her six flailing legs dislodges loose sand grains and she slips downward. This commotion, of course, calls the ant-lion into action. Lest the ant make some upward progress, the ant-lion will toss volleys of sand up at her to thwart her climb. The ant-lion repeatedly releases and repositions her so that it can insert its jaws into her body; then, it will inject a narcotic that will make her stop squirming. Eventually, she succumbs and disappears into the ant-lion’s lair.
Is this all a tragic tale of hapless ants being drawn inexorably toward the jaws of death? No, ants are smarter than that. More research by Nicholas Gotelli shows that ants learn to avoid the ant-lions’ minefields. Even when offered their favorite food — canned tuna — in and among ant-lion traps, ants forgo the treats and dodge the sand traps they recognize as threats. And ant-lions also recognize when fake prey have “fallen” into their traps; it’s only the genuine struggling insect that attracts their attention.
All this may explain why all of us playful kids — of all ages — have trouble coaxing ants into ant-lion nests or coaxing ant-lions out by wiggling pine needles in their sand pits. Insects are way smarter than we give them credit for.
Elizabeth Farnsworth is Senior Research Ecologist with New England Wild Flower Society, and definitely a playful kid at heart.
Earth Matters, written by staff and associates of the Hitchcock Center for the Environment at 845 West St, Amherst, appears every other week. For more information go to www.hitchcockcenter.org, call 256-6006 or write to email@example.com.