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Why Do Gnats Swarm?

Gnat swarm
So many gnats! (Image credit: Shutterstock)

There's almost nothing worse than biking along a canal and pedaling headlong into a swarm of gnats — a loose name for mini mosquito look-alikes called midges — clustered near the water. These nearly invisible gatherings of tiny flies pop up around streams, fields and country roads … but why do gnats insist on crowding together in the same tiny air space?

The answer is that it makes it easier for males and females to do the hanky-panky, so to speak, said Gregory Courtney, professor of entomology at Iowa State University.

"The bottom line is most male flies swarm," Courtney told Live Science. "It's an important mechanism for the two sexes to find each other." Male gnats always swarm in order to attract females, but the location of that swarm depends on their surroundings. [What Makes Grasshoppers Swarm?]

"Typically, swarms are going to form around particular objects or visual markers — that might be a ripple above a stream or a fence post along the roadside," he said. Swarm markers include various objects that contrast with surrounding landscape, which, according to Courtney, makes it easier for females of the same species to see the swarm. Oftentimes, swarms occur in sunlit areas, he said, so when the sun's rays change position, the swarm will likely move with it.

There is a downside to gnats' midair sexual antics. It makes them visible to predators, Courtney said. Predatory dragonflies, for example, will fly back and forth across swarms, indulging in multiple rounds of in-flight meals.

The definition of a swarm is as fluid as a swarm itself. "It's any aggregation of individuals for the purposes of attracting mates," Courtney said. It could be half a dozen individuals congregating for some afternoon delight, or millions of individuals converging at once. Courtney pointed to Lake Victoria in East Africa as an example: "Sometimes a swarm is so large you can see it as a huge cloud out over the water."

Females never swarm. Instead, they enter their male counterparts' roiling mass only to mate. Courtney admits it's a real frustration trying to track what happens to the female after she flies into the fray. "I can easily spot a female flying over the river because her flight pattern is so different from a male's," he said. "But once she enters the swarm, I lose her."

This can make research challenging; Courtney can determine where in the swarm a female enters, but then can't determine if she moves around to find an ideal male or starts mating with the first male she encounters.

Considering that midges are usually short-lived — some live no more than a few hours, others a few days — swarming can be the most important event in males' brief lives.

"There's been a lot of interest whether certain places within the swarm are more advantageous for males' mating and if there's competition among males to get to those spots," Courtney said. He thinks the tops or margins of swarms could be most ideal, but where the prime position in a swarm is depends on the species, he said. [What If All of Earth's Insects Keeled Over?]

Most swarms follow a specific pattern of movement, like a distinct back-and-forth or rotating pattern, Courtney said. Adherence to those patterns prevents individuals in them from constantly bumping into each other. Courtney also said that flies have adapted to be agile midflight. "True flies, or [the order] Diptera, only have two wings, and their hind wings have been modified into gyroscopic organs called halteres," he said. Halteres provide feedback about how flies' bodies rotate in the air, and according to Courtney, it's thought that this particular balancing organ is directly related to swarming behavior.

But even though these swarms can be annoying to people, it's not worth trying to minimize midges' swarming behavior in, say, your backyard, Courtney said. That's because, unlike mosquitoes, most species of midge don't bite. With the exception of biting midges (aka "no-see-ums" for Cape Cod and New England folks), gnats don't swarm around humans on purpose.

In other words, it might just be easier to keep your mouth shut when you go biking.

Original article on Live Science.

Live Science Contributor

Aylin Woodward is a science reporter who covers space exploration, anthropology, paleontology, physics and material sciences. She has written for Business Insider and now reports at The Wall Street Journal. She graduated from the University of California, Santa Cruz science communication Master's program, and earned a bachelor's degree from Dartmouth College. She received a National Science Foundation Graduate Research Fellowship in 2016 for work focused on hominin bipedalism.