For most of the year, the Karner blue butterfly exists as a tiny, dimpled white egg. But as winter days lengthen into spring, the life inside begins to stir, and soon a pale green caterpillar emerges. It feeds, then cloaks itself in a chrysalis. And then, for one brief week, the Karner blue is on the wing. Or, a wing and a prayer. For decades, the butterfly with the fleeting appearance was on the verge of becoming a fleeting species as its oak savannas and pine barrens disappeared. Now, with efforts to restore its preferred habitat and raise the butterflies in captivity, the Karner blue’s future is decidedly more upbeat. The rare blue butterfly lives for the wild blue lupine. When Karner eggs hatch in April, lint-sized caterpillars emerge to feed on the leaves of the newly sprouted plant. Nothing else will do. And when the mature butterflies are looking to lay eggs for a second brood, again, they’ve only got eyes for lupine.