Quirks & Quarks

Do fireflies glow just at night or do they glow during the day too?

Fireflies mostly only glow at night because that's when the males are trying to get the attention of female fireflies, according to entomologist Dr. Sandy Smith.
Fireflies, also known as lightning bugs, thrive in the hot weather, which may be why we're seeing more of them this year. (Jessica Lucia)

This week's question comes from Shirley Wildenbeest in Kingston, Ont. who asked: "Do fireflies glow just at night or do they glow during the day too? And one more question, are there certain conditions or certain times during the summer when they are more actively blinking?"

Here's Dr. Sandy Smith, Professor of Entomology and Forest Health at the University of Toronto's response:

So the answer to that question is, of course, it depends. And there's lots of variations. There's at least 23 species of fireflies in Ontario. Most of them glow at night because they're nocturnal, and that's usually the males that are involved in mating. Males are glowing and pulsing and flashing lights in a sequence that is very species-specific so that females can identify them as from the same species. That way there's no missed mating or wasted energy trying to court a female that's not in your species.

A few species are diurnal in the daytime and so they don't usually glow, although most glow worms (as they call them in Europe), or lightning bugs as we call them here, during one of the maturation stages do glow. It even can be the caterpillars or the larvae of the firefly that glow. So the answer is, it depends.

[Responding to the second question], fireflies over winters are larvae in sort of wet marshy areas and rotting wood. That's where they're predacious on other insects and slugs. But the adults emerge out in the warmer late spring, early summer, when it's warm and humid. And that's when we usually see the flashing with the adults, the nocturnal species; we're probably more aware of those. It's usually June and July when they're mating and very active as adults.

The adults only live a few months at most and most of their life cycle is spent as larvae in the soil because once they've attracted males in through the flashing signals, they mate, they lay eggs in the soil and then the life cycle continues as larvae to the following year.