Researchers in the UK have turned some highly specialized radar equipment to the skies to track a staggering volume of material—3.2 kilotons of it—as it transits the skies of southern England. The material in question? Insects, about 3.5 trillion of them each year. While smaller insects seem to drift on the prevailing wind, larger ones appear to undergo a directed, seasonal migration.

The study relied on a combination of high-tech and old-fashioned hardware, shown above and below. The old-fashioned equipment was a traditional insect-catching net, albeit one that was sent aloft trailing below a miniature blimp. This was needed to pick up smaller insects—below 10mg in body mass—which couldn't be tracked using the radar. The radar was a special piece of equipment, called a vertical-looking entomological radar.

It's impressive hardware. For anything above 10mg, the radar could record "body mass, flight altitude, aerial density, displacement speed, displacement direction, and flight heading." The equipment could do all this up to about 1.2km in altitude, allowing it to catch any high-flying insects. The equipment was set up in three locations in the southern UK and sampled the movement of insects for a full decade to produce the data analyzed in this new paper. During that time, the radars tracked more than 1.8 million individual insects, which were used to extrapolate general trends.

The numbers that come out are pretty staggering. Over the decade the study ran, the researchers recorded a mean of 3.37 trillion insects traveling above the study area. Collectively, this added up to 3,200 tons of insect.

In terms of numbers, the vast majority of them fell into the "small" category, meaning they couldn't be picked up by the radar. The larger insects made up for numbers simply through mass. Although they accounted for less than half a percent of the raw numbers, they were nearly 20 percent of the total mass of organisms that traveled above the site.

The larger insects were also more interesting in terms of how they traveled. Smaller ones tended to go during the daytime, simply moving on the prevailing winds, which would tend to push them to the northeast. By contrast, large insects appeared to be involved in seasonal migrations, moving north in the spring and back south in the autumn. They also tended to migrate at night. The researchers also found that these insects preferentially moved at times when the winds would help them on their way—even if those winds were only present well above ground level (a situation more common at night).

The travel speed of these larger insects indicated that they weren't simply passively riding the wind. They reached speeds of between 30 and 60 kilometers an hour, which suggested that they were actively flying along with the wind. The authors note that, in just a few hours, these insects would be able to cover over 200km.

Within the error of their measurements, the authors couldn't detect a net flow of insects. In other words, as far as they could tell, the same number of insects moved south in the autumn as had gone north in the spring. While there are substantial errors in these measurements (up to 200 tons a year), this means that the flow of insects isn't clearly creating a flow of nutrients.

Still, the sheer volume of insects is striking. The authors note that a major migration that many people are aware of—UK songbirds heading to Africa for the winter—only involves 415 tons of animal in transit. Meanwhile, a much larger migration was apparently flying under the radar—but not the fancy vertical-looking entomological radar.

Science, 2016. DOI: 10.1126/science.aah4379 (About DOIs).