As summer heats up and humidity thickens the air, Minnesota’s annual spectacle of fireflies arrives. The flashes of greenish to yellowish light flicker at the edge of meadows and moist woods. Or you may catch a solo blinker so brief and intermittent, you wonder if you imagined it.

Fireflies, also known as lightning bugs, are neither fly nor bug. They are a part of the beetle family (Lampyridae) and in the aptly named Photuris genus.

They flash across every continent but Antarctica and in every U.S. state except Hawaii, according to the Xerces Society for Invertebrate Conservation. Minnesota has about six species of fireflies, which light up from late June into July and sometimes August.

The flashes often work like a Morse-code mating dance. Males blink a pattern as they fly and hope a female, often perched on foliage, will flash an answer.

A firefly in Great Smoky Mountains National Park (Radim Schreiber)

The added allure of seeing synchronous fireflies, which have the ability to light up in unison in parts of the Great Smokies in Tennessee and South Carolina’s Congaree National Park, has become so popular that there’s a lottery to reach viewing areas.

Fireflies produce light through bioluminescence, the same phenomenon observed in some fungi, underwater jellies, plankton, glow worms and fish. The chemical luciferin mixes with oxygen and luciferase in the lanterns on the bugs’ abdomens to create the cool glow.

Scientists think fireflies can regulate flashing through their intake of oxygen.

A wet spring might give this year’s firefly show a slight advantage, but researchers say firefly numbers are shrinking overall. Causes are thought to include loss of habitat, light pollution, pesticide use and climate change.