Video: Watch a fly inflate its head to break through a wall and escape

Loitering with intent: an adult Cacoxenus indagator hanging around the nests of a red mason bee, waiting for a chance to lay its eggs inside (Image: Erhard Strohm) Inside the red mason bee’s nest: the entrance is over to the right, and the wall closest to it is visibly convex. A single bee egg is attached to a bundle of pollen and nectar (Image: Erhard Strohm)

Species: Cacoxenus indagator


Habitat: hanging around bees’ nests across Europe

You’re walled up in the dark and all your food has gone. You have to escape. But to get out you have to smash your way through a stone wall with your head – without even knowing if you’re going in the right direction.

It might sound like a scene dropped from Saw XXIII, but this is how the fly Cacoxenus indagator begins its adult life. In the same situation humans would struggle, to say the least, but the fly knows exactly what to do.

Busy burglarising bees

The story begins in spring, when female red mason bees are building their nests. Unlike honeybees, they are solitary, and to save on the hard graft of construction they rely on other species to dig holes for them. Thin tubes burrowed in trees by wood-boring beetles are a favourite, and playful zoologists have even persuaded the bees to nest inside drinking straws.

Each nest begins as a simple tube. The female drops a small supply of pollen and nectar at the furthest end, and glues a single egg to it. Then she retreats a short distance and rapidly builds a wall of mud to seal the passage. She repeats this several times, filling the tube with a line of “brood cells”, each containing a single egg and a food cache.

Over the next few months the eggs hatch and the larvae feed on the pollen and nectar. Then they spin a cocoon and metamorphose into adults, before hibernating through the rest of the year, emerging in the next spring.

At least, that’s the plan. Cacoxenus indagator has other ideas, which involve requisitioning those bundles of nectar and pollen. Adult flies loiter at the entrances of nests and will lay up to eight eggs in a single brood cell while it is being stocked. When the fly larvae hatch they feed on the nectar and pollen before pupating – starving their bee larva cellmates in the process – to emerge from their cocoons as adults the following spring.

But at this point they run into a brick wall, almost literally. The mud walls are 2 to 6 millimetres thick, and they have dried hard. Each newly adult fly is only about 3 millimetres long, so the wall it must break through may be thicker than its body is long.

Hydraulic woodpecker

The fly’s solution, says Erhard Strohm of the University of Regensburg, Germany, is to meticulously headbutt its way out.

Some of the flies, he has found, have it easy: at least one bee in the nest usually survives to adulthood, and breaks out of the nest using its powerful mandibles, passing through as many brood cells on the way as it needs to. This leaves a clear route to freedom for around two-thirds of trapped flies. But the others are on their own.

To do so they must first pick the correct direction, using the clues left behind by the mother bee: within each cell, the wall nearest the exit is convex and the other is concave. Strohm tested 20 flies in artificial nests, and found that 19 of them focused their efforts on breaking through the correct wall.

Strohm then filmed flies trying to escape from brood cells. He found that each pushed its head into a small crevice in the dried mud, then broke pieces off. How? By inflating its head (see video, above).

It manages this trick by pumping haemolymph – the insect equivalent of blood – into a pouch on its head called the ptilinum. This is used in many flies for breaking out of their cocoons, but Cacoxenus indagator has taken it a step further. “The pumping is quite strong, so it can exert plenty of pressure,” Strohm says. The hole the fly makes is often rather small, but its body is still soft, so it can squeeze its way through.

It’s a weird escape plan, but an effective one. Of 98 flies Strohm observed trapped in brood cells, not one failed to escape.

Journal reference: Physiological Entomology, DOI: 10.1111/j.1365-3032.2010.00764.x

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