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We call it Tiszavirágzás, or Tisza blooming. Every year from late spring to early summer, a natural spectacle transforms Hungary's Tisza River. Villagers come to marvel at the "flowers" blooming on the river's surface—millions of long-tailed mayflies. Rising in huge clouds, they take flight, reproduce, and perish, all in just a few hours.

My father, who grew up in a village not far from the river, often told me how the fishermen and ferrymen seemed to know from experience when the mayflies would appear. I sought out those people to pinpoint the exact place and time to photograph the sudden mayfly masses.

To keep alert for erupting mayflies, I enlisted several spotters armed with cell phones.

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The "ephemeron," Aristotle called the short-lived mayfly, which numbers 2,000 species worldwide. With males measuring up to five inches from head to tail, the Tisza's Palingenia longicauda is Europe's largest mayfly.

Shortly after mating, females lay eggs on the river's surface. The eggs drift to the bottom and after 45 days hatch into larvae, which dig tunnels forming dense colonies up to 400 per square foot. After three years larvae break for the surface where females molt once and males shed twice: first into a brief subadult stage then again minutes later into adulthood. After both sexes have fully matured, mayflies have roughly three hours before they die.

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During the mating period the river's surface explodes to life. Adult males flutter above the water, their wings a whir. There is no courtship in the mayfly repertoire. Reproduction is often a forcible act with up to 20 males simultaneously going after a lone female. An eager male might also lie in wait atop the skin of a female that has yet to shed.

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VIDEO Photographer and biologist József Szentpéteri talks about what happens when mayflies, during their fleeting life cycle, swarm into town. AUDIO (recommended for low-speed connections) RealPlayer  WinMedia

A hungry kingfisher surrounded by mayflies makes a colorful e-greeting.

Today, Hungary's Tisza River and its tributaries are the only place in the world where you can see the spectacular swarms of long-tailed mayflies, Palingenia longicauda. But a century ago the insects flourished in Europe's lowland rivers (except those in the Mediterranean and Scandinavian regions). So what caused them to disappear? Ecologists say it was a combination of water pollution from heavy industries and riverbank re-engineering, which destroyed the mayfly larvae's natural habitats. After communist factories closed in the early 1990s, many rivers in Eastern Europe experienced a drop in pollution levels. Efforts are now being made to restore these once-tainted rivers: One strategy involves harvesting hibernating eggs from the Tisza, then transplanting them into other European rivers. Some conservationists, however, feel that reintroduction efforts should be secondary to keeping the mayflies that already exist on the Tisza healthy.

—Julie Cederborg