The more you know about monarch butterflies, the more extraordinary they seem. Their life cycle — the adaptive web of behaviors they have evolved — is almost unbelievably complex. They migrate en masse (this month, as a rule) from all across the Midwest and Northeast to just a few high altitude sites in Mexico, where they winter, sheltering in profusion beneath a canopy of fir trees, from whose trunks they draw the residual heat they need to stay alive. Winter past, they migrate thousands of miles northward, where they lay their eggs on milkweed plants, the only plant a monarch caterpillar can eat.