Tiny backpacks for dragonflies track their brains in flight
The brain of a dragonfly has to do some serious calculations — and fast — if it hopes to nab a mosquito or midge in midair. It has to predict the trajectory of its prey, plot a course to intersect it, then make adjustments on the fly to counteract any evasive manoeuvres. Neuroscientist Anthony Leonardo created the tiny dragonfly backpack above to study how circuits of neurons do these computations.
The backpack weighs 40 milligrams, about as much as a couple grains of sand, equal to just 10 percent of the dragonfly’s weight. Electrodes inserted into the dragonfly’s body and brain record the electrical activity of neurons, and a custom-made chip amplifies the signals and transmits them wirelessly to a nearby computer.
One of the trickiest design challenges was how to power the chip without adding so much mass that the insects couldn’t get off the ground, says Leonardo, who’s based at Howard Hughes Medical Institute’s Janelia Farm Research Campus in Ashburn, Virginia.
He and collaborators at Duke University and Intan Technologies came up with a clever solution based on the same technology found in the RFID key card access system used in many office buildings. There, a reader, usually a small pad next to a door, emits radio waves to create a magnetic field. When a key card gets close enough to the reader, the magnetic field induces a current that powers a chip inside the card, enabling it to transmit a code to unlock the door.
The two long antennae on the dragonfly backpack harvest radio waves and power the chip in a similar way. Eliminating the need for a battery on the backpack was the key to keeping the weight down.
Getting dragonflies to hunt inside the lab turned out to be a little tricky too, Leonardo says. In a plain white room, the insects exhaust themselves trying to escape. So the team installed turf on the floor, installed a small pond, and covered the walls with a scene that evokes a springtime meadow.
In their experiments, the researchers release fruit flies and watch the dragonflies take off from a perch and catch them. Eighteen high-speed infrared video cameras positioned around the room capture every move as a dragonfly closes in on its prey and launches its body upwards, curling its hairy legs inward to form a sort of basket trap.
As the dragonfly hunts, the backpack captures the firing of neurons Leonardo thinks play a crucial role in guiding it towards its prey. “We know a lot about their anatomy,” he said. “They gather input from visual parts of the brain and send axons down to the motor neurons that move the wings.”
The question that fascinates Leonardo is how those neurons and others transform information about the visual scene into a plan of action, and how they continuously update the plan as the dragonfly and its prey move through space. All animals do this type of transformation, from a centre fielder running down a fly ball to a lion running down a gazelle. But a neuroscientist can’t exactly study those situations in the lab.
"The dragonfly is a convenient and beautiful and elegant means to an end," Leonardo said.