When fish flies swarm: 5 things to know

In Gimli, Man., vast numbers of clinging critters that locals call fish flies are heading out for the final stage of their lives.

The insects are actually burrowing mayflies, University of Manitoba entomologist Terry Galloway said.

"They are quite a remarkable animal," he said. "If it wasn't for the big pile of stinking mass on your property, you have to admire these things. They really are extraordinary insects."

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Most of their life is spent underwater

Galloway said the mayflies begin life in the soft substrate at the bottom of lakes and rivers, and they can live there for two years or more.

They have gills

Although they make little burrows (thus their names), the mayfly's marine life sounds a lot like that of a fish. They have "really lazy" gills along their abdomen that are used to create a current that draws water into their subaquatic home, Galloway said.

"That's how they get fresh water, and they can extract the oxygen from the water as it flows over these gills and over their bodies," he said.

They also feed on organic materials.

Skin shedders

Once the flies emerge and cover a community, people will probably notice there are two different colours. They are unusual in the insect world, one of a group of insects that moult as adults, Galloway said.

When the mayflies first emerge they are creamy coloured sub-adults and said they can't reproduce, said Galloway. But once they drop their skin, the flies turn a reddish brown and are ready to make more mayflies.

They don't have a mouth

Since mayflies get their fill of organics and oxygen for years before they emerge, once they are flying around, they have no need for food.

During their entire adult lifespan, they don't eat.

"They don't even have developed mouth parts at all," Galloway said.

A taste of the sky and then they die

Although the full lifespan of a mayfly can be years, the adult stage is brief.

"The adults don't live for long, just a few days," Galloway said.

They may not last long, but the mayflies in Manitoba make their presence known. 

There are mayflies every year, but Galloway said environmental factors, such as a lack of storms or wind when eggs are laid, can lead to different numbers when the adult season comes around.

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