Great article in today's NYT about the Monarch Butterfly......

NYT  September 21st.

Dave Taft

Each autumn, thousands of monarch butterflies sweep southbound through New York City in one of the world’s most famous migrations. A journey of more than 2,000 miles will eventually take them to central Mexican fir forests, where they will winter on south-facing slopes in numbers large enough to break tree boughs. It is a migration substantially unlike those of birds.

For one, no monarch butterfly leaving New York City will see the Manhattan skyline again, unlike the returning tree swallow or New World warbler. There may be two or three generations between the monarch you see this fall and the one returning in the spring.

The monarchs fluttering through May’s flowering meadows come from eggs deposited by adult butterflies who spent the Mexican winter in a state of hibernation-like torpor, then flew north with the first warming weather and, using their last bit of energy, laid eggs on the milkweeds of Louisiana, Texas and Florida. Those butterflies then laid their own eggs as they traveled north.

How monarchs (Danaus plexippus) learn their migration routes is still a great mystery. No monarch butterfly lingers near its eggs long enough to meet its caterpillar progeny, let alone teach the attractive but utterly earthbound caterpillars to fly. The parent also plays no role in instructing its offspring on the proper route. The itinerary is passed along through generations as preprogrammed information. It is a migratory map, hard-wired into a minuscule and ancient insect brain, and a programming feat scarcely matched by all our modern advances in the age of computers.

Consider, too, that this memory is held by an organism that will liquefy and completely reconstitute during pupation, changing from the fleshy, leaf-chewing caterpillar into an ethereal, nectar-sipping, winged adult — all the while retaining this critical migratory information.

The final generation of butterflies that emerges in the waning days of our Northern summer and fall postpone reproduction — they leave the chrysalises in what is called reproductive diapause. This is triggered by factors including shortening days, cooler temperatures and, potentially, the chemical compounds found in the late-season milkweeds the caterpillars feed upon.

With yet unripe reproductive organs, all their energy is channeled into the long journey south.

Migration is remarkable, as interesting conceptually as it is in the cold light of scientific inquiry. It is also borderless — and offers a critical lesson in global conservation. Monarchs are threatened by pesticides, changes in weather patterns and deforestation throughout their extensive range.

But migration is also wonderfully participatory — you can watch eons of evolution flying past in any of the five boroughs’ flowering meadows on windy autumn days. Parks along the eastern shore of Staten Island, or the southern coasts of Brooklyn and Queens, often host truly remarkable numbers of insects winging their way southbound in early fall.