The monarch butterfly's journey

There is a butterfly alive right now, on milkweed plants across the northern United States and Canada, that will migrate roughly 2,000 miles this autumn to mountain forests in central Mexico that it has never seen. It will arrive at the same mountain range its ancestors arrived at, within the same weeks of the year, following the same route. No one taught it. Nothing in its lifetime has pointed it there.

The monarch butterfly (Danaus plexippus) has always been described as miraculous. It is more specific than that.

The monarch overwintering in Mexico this November is not, in any useful sense, the same individual that flew north from Mexico last March. It is three or four generations removed. A spring butterfly leaves Mexico, flies north to Texas, lays eggs, and dies. Those eggs become caterpillars, then butterflies, in May. Those butterflies continue north, lay their own eggs, and die. Their offspring do the same through June and July. What is hatching from eggs on milkweed right now, in the last week of July, is the caterpillar that will become the butterfly that does not die in six weeks. This generation will live for seven to nine months. It will delay reproduction entirely until next spring. It will store fat for a journey it has not yet taken. The same genetics; a completely different life.

The destination requires solving a navigation problem. In 2009, Christine Merlin, Robert Gegear, and Steven Reppert at the University of Massachusetts published an experiment in Science that located the monarch's compass. It is not in the brain. It is in the antennae.

The monarch uses a time-compensated sun compass: it tracks the position of the sun to maintain a consistent southwestward bearing through the day. Because the sun moves across the sky, the compass needs a clock to compensate, "toward the sun" means a different direction at 9am than at 3pm. The clock that does this compensation is in the antennae. When Merlin and colleagues covered a monarch's antennae with opaque paint, the butterfly lost its orientation. When they used transparent paint, blocking touch and airflow, not light, navigation was intact. The light reaching the antennae resets the clock. The clock calibrates the compass. The compass points south.

None of this is visible in the caterpillar eating milkweed right now. It looks like every other caterpillar. Striped yellow and black and white, working methodically through the leaves. But in the cells of that caterpillar are instructions for a seven-month life, for reproductive delay and fat accumulation, for an antenna-based navigation system that will solve, without error, the problem of finding a specific range of mountains in Michoacan, Mexico, at the right altitude, among the oyamel firs, at the right time.

Millions of butterflies will crowd those trees in November. Their weight will bow the branches down. Every one of them will have arrived there for the first time.

There is probably milkweed within a few miles of wherever you are in the eastern United States right now. If there are monarch caterpillars on it, one of them may be this generation, the one with the extended life, the one that does not behave like a summer butterfly, the one with the time-compensated clock in its antennae and a destination it has never seen.

Rewyld is built on the observation that the living world is doing something specific, in this place, on this day, and that paying attention to it is its own thing. Late July is not a quiet time of year. It is when the impossible is getting started.