PETER MENZEL / SCIENCE PHOTO LIBRARY PETER MENZEL / SCIENCE PHOTO LIBRARY
A swarm of North American monarch butterflies, Danaus plexippus, gathered on trees in Rosario, Mexico. Aside from a small proportion who overwinter as pupae in the north, these insects are remarkable for the 3000 kilometre annual migration they undertake. After emerging during the summer in the Great Lakes region on the Canadian border they fly southward, ahead of the advancing snows, to winter in Florida or Mexico. Here they congregate in their millions on trees & houses until flying north again in the following spring. Many die on the way, but not before they have produced offspring which continue on to the Great Lakes.
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