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.