Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszakurt, Hungary, June 14, 2016. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh TPX IMAGES OF THE DAY

A photographer takes pictures of long-tailed mayflies (Palingenia longicauda) at Tisza river near Tiszakurt, Hungary, June 14, 2016. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszakurt, Hungary, June 14, 2016. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh

A man holds a long-tailed mayfly (Palingenia longicauda) in the Tisza river near Tiszakurt, Hungary, June 14, 2016. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh

A man holds a long-tailed mayfly (Palingenia longicauda) in the Tisza river near Tiszakurt, Hungary, June 14, 2016. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszakurt, Hungary, June 14, 2016. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszakurt, Hungary, June 14, 2016. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszakurt, Hungary, June 14, 2016. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszakurt, Hungary, June 14, 2016. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszakurt, Hungary, June 14, 2016. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszainoka, Hungary, June 18, 2015. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Bernadett Szabo

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszainoka, Hungary, June 18, 2015. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Bernadett Szabo

A boy swims with long-tailed mayflies (Palingenia longicauda) at Tisza river near Tiszainoka 135km (84 miles) southeast of Budapest, June 23, 2013. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh (HUNGARY - Tags: ENVIRONMENT ANIMALS SOCIETY)

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszainoka, 135km (84 miles) southeast of Budapest, June 23, 2013. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh (HUNGARY - Tags: ENVIRONMENT ANIMALS SOCIETY)

A photographer takes pictures of long-tailed mayflies (Palingenia longicauda) at Tisza river near Tiszainoka 135km (84 miles) southeast of Budapest, June 23, 2013. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh (HUNGARY - Tags: ENVIRONMENT ANIMALS SOCIETY)

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszainoka, 135km (84 miles) southeast of Budapest, June 23, 2013. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh (HUNGARY - Tags: ENVIRONMENT ANIMALS SOCIETY)

Long-tailed mayflies (Palingenia longicauda) fly over the surface of the Tisza river near Tiszainoka, 135km (84 miles) southeast of Budapest, June 20, 2012. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh (HUNGARY - Tags: ENVIRONMENT ANIMALS SOCIETY TPX IMAGES OF THE DAY) FOR BEST QUALITY IMAGE ALSO SEE: GF2E8730PB901

Long-tailed mayflies (Palingenia longicauda) fly over the surface of the Tisza river near Tiszainoka, 135km (84 miles) southeast of Budapest, June 20, 2012. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh (HUNGARY - Tags: ENVIRONMENT ANIMALS SOCIETY)

A man holds a long-tailed mayfly (Palingenia longicauda) at Tisza river near Tiszainoka, 135km (84 miles) southeast of Budapest, June 20, 2012. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh (HUNGARY - Tags: ENVIRONMENT ANIMALS SOCIETY)

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszainoka, 135km (84 miles) southeast of Budapest, June 20, 2012. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh (HUNGARY - Tags: ENVIRONMENT ANIMALS SOCIETY TPX IMAGES OF THE DAY)

A man holds a long-tailed mayfly (Palingenia longicauda) at Tisza river near Tiszakurt, 135km (84 miles) southeast of Budapest, June 12, 2011. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season, from late spring to early summer every year. REUTERS/Laszlo Balogh (HUNGARY - Tags: ANIMALS SOCIETY)

A long-tailed mayfly (Palingenia longicauda) rests on the face of a Hungarian man, Balazs, at the Tisza river near Tiszakurt, 135km (84 miles) southeast of Budapest, June 12, 2011. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Lasz (HUNGARY - Tags: ANIMALS SOCIETY ENVIRONMENT IMAGES OF THE DAY)

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszakurt, 135km (84 miles) southeast of Budapest, June 12, 2011. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh (HUNGARY - Tags: ENVIRONMENT ANIMALS SOCIETY)

Long-tailed mayflies (Palingenia longicauda) mate on the surface of the Tisza river near Tiszakurt, 135km (84 miles) southeast of Budapest, June 12, 2011. Millions of these short-lived mayflies engage in a frantic rush to mate and reproduce before they perish in just a few hours during "Tiszaviragzas" or Tisza blooming season from late spring to early summer every year. REUTERS/Laszlo Balogh (HUNGARY - Tags: ENVIRONMENT ANIMALS SOCIETY)