From the Lacrosse, Wisconsin office of the National Weather Service, via reader Gregory, we see Doppler radar capturing a massive mayfly emergence on June 23 (a description of how Doppler radar works is here). I believe the species is the giant mayfly, Hexagenia limbata, though I may be wrong.
On Saturday evening, June 23 2012, a massive mayfly emergence occurred along the Mississippi River beginning just after 9 pm. By late evening, mayflies were swarming in La Crosse, La Crescent, and points up and down the river. While the emergence of mayflies from their river bottom mud dwelling can occur at various times through the warm season, this particular event was one of the best seen on radar yet. In the radar time lapse loop from 9 pm to just after 1030 pm (below), the yellows and oranges indicate a large magnitude of airborne mayflies.
Also very evident was the northward track to the mayfly radar ‘echo’. This movement was due to the south wind direction over the area at emergence time. The radar would indicate the bugs were carried north, off of the river, into Blair and Taylor Wisconsin. The radar beam over these locations are detecting mayflies at over 3000 feet above the ground! Their existence was confirmed on the ground north into Trempealeau county near Galesville.
There is more information at the site.
Mayflies are in the order Ephemeroptera, meaning, in effect “short-lived winged things.” (They aren’t really flies, which are in the order Diptera.) The Freshwater Blog details their life cycle; here’s a short extract:
A mayfly’s life cycle starts with the males forming a swarm above the water and the females flying into the swarm to mate. The male grabs a passing female with its elongated front legs and the pair mate in flight. After copulation, the male releases the female, which then descends to the surface of the water where she lays her eggs. Once mated she will fall, spent, onto the water surface to lie motionless, with her wings flat on the surface, where fish pick them off at their leisure. The male fly rarely returns to the water but instead he goes off to die on the nearby land.
Because they live only a day, they cannot feed, as their mouthparts are vestigial (explain that, creationists!). In the species Dolania americana, females live less than five minutes as adults, the shortest life span of any adult insect. What a life—if you can call it that!
Here are some pictures taken the next day in Wisconsin, also from the site:
More from the Freshwater Blog:
Some species exhibit great synchronicity in their hatching. The North American species Hexagenia limbatahatches in huge numbers from the Mississippi every year. The total number of mayflies in this hatch are estimated to be around 18 trillion – more than 3,000 times the number of people on earth. The newly emerged insects are attracted to lights in riverside towns and villages and the local authorities deploy snow clearing vehicle to remove their rotting corpses. Ironically, what is seen as a nuisance in America is seen as a gift in Africa. Locals around Lake Victoria gather adults of the mayfly Povilla adusta together with Chironomid midges to make a type of patty called ‘Kungu’. This protein rich food stuff is an important part of their diet.
Station KARE from Minneapolis/St. Paul reports:
The La Crosse, Wis. office of the National Weather Service (NWS) says this year’s mayfly hatch on the Mississippi River was so prolific that it created a bow echo on radar, similar to one that would be made by a significant rain storm. A NWS employee went out after his shift and captured some amazing images of the short-lived pests covering street lights, gas pumps, buildings, stairs — nearly anything in their path.
The mayfly hatch was a problem up and down the Mississippi. Police in the town of Trenton say a large hatch of mayflies may have triggered a three-vehicle crash on a Wisconsin road.
and adds a picture of the insects at a gas station: