A Staff Report from the Straight Dope Science Advisory Board

Why do fireflies only glow when flying upwards?

September 4, 2000

Dear Straight Dope:

Summer is here and the fireflies are out. I've noticed that when fireflies glow, they are almost always flying upwards. You rarely see one start glowing while it is descending. Why do fireflies behave this way?

SDStaff Doug replies:

I admit, I've never heard this question before.

Good observation, but the only reason it seems that way to you is that you've only been seeing the males of ONE species of firefly, and there are about 140 species or so in the U.S. Every different firefly species has its own aerial display pattern, and the one you've got in your area, Photinus pennsylvanicus (the most common species in the U.S.), just happens to flash in a short upwards motion. That's how female fireflies tell males of the different species apart, and know whether they should respond or not. The females just sit on the ground or vegetation, and respond with "come hither" flashes to the males of the right species. There is even a predatory species whose females give "come hither" flashes to ANY passing male, then eat the unsuspecting male when he's crawled close enough.

Basically, if you go anywhere with different species of fireflies, you'll see any number of different flash patterns, but none exactly like the "upstroke" one you're familiar with.

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