Duluth residents need only to crane their necks toward the brightest lights in the city to see evidence of it.

The mayfly hatch has arrived.

With that arrival comes a swarm of millions of the winged insects. They only hang around for a few days, which means there's one thing on each of their minds: finding a mate.

"Their sole purpose is to mate," said Heidi Rantala, an aquatic ecologist with the Department of Natural Resources.

There isn't much else they can do. They don't have a mouth. For the few days after they hatch and depart from their aquatic dwellings, they spend the entirety of their terrestrial existence searching for a partner.

The species typically likes to synchronize its mating, which is why they show up in such abundance.

"They emerge in mass because the adults are short-lived," said Jeff Hahn, an extension entomologist with the University of Minnesota. "That's why we get these spectacular hatchings. The immatures molt into adults."

After they're done, the females leave to lay their eggs, while the males become food for predators.

Mayflies like Minnesota. The plethora of rivers, ponds and lakes serves a healthy population of many variations of the insect.

"Different mayflies like different habitats," said Rantala. "Some like soft sediments, or others look for the bottom of lakes and rivers, and some like to hang onto vegetation. We've got a diversity of different waterbody types, so you would expect a variety of them."

Mayflies spend almost their entire life underwater. During their nymph stage, they are fodder for many aquatic predators. When they breach the water's surface and fly above ground, they're the preferred meal for birds, bats and other insects.

Mayfly hatches have in the past produced extraordinary sights, both to the naked eye and on the Doppler radar. While some videos of the insects blanketing gas stations might leave viewers in horror, mass hatches so big they appear as storm systems on the National Weather Service radars leave many in awe.

However, instances like these leave wildlife ecologists smiling. Millions of mayflies mean a healthy ecosystem. One way the Minnesota DNR classifies the health of waterbodies is by the mass hatches of mayflies.

"It really is a good thing. It's a good indicator of ecosystem health," said Rantala. "Mayflies are indicators that are associated with the quality of water."