You’ve probably seen a hoverfly, but you may have thought you were looking at a wasp or a bee. Hoverflies are harmless, but they have evolved to mimic stinging insects as a way to scare away predators.

Hoverflies provide two big ecological benefits. As larvae, they defend gardens and farms by eating aphids. As adults, they pollinate flowers as they eat nectar and pollen.

“I don’t think you could create a more useful insect,” said Dr. Chapman.

Scientists had clues that some species of hoverflies migrate, but they knew little about the timing or the scale of their movements.

To tally the hoverflies migrating through southern England, Dr. Chapman and his colleagues brought one species, the marmalade hoverfly, into the lab and bounced radar off it. They determined the distinctive radar signature of the insect, and then searched for it in scans made by their network of instruments.