The poem opens with a striking image: a winter forest looks like a house emptied by burglars. The leaves are gone, the branches are bare, and everything seems stripped. The speaker, though, finds something appealing in that openness and hopes to be "this free and open" in old age. The "fire stolen from the sun" suggests the lost warmth and color of autumn.
Then a question breaks in: where have all the caterpillars gone? The rest of the poem answers it. They haven't vanished. Some are dormant, protected by glycerol in their blood so ice can't form. Some lie dead in the leaf litter, wings neatly folded. Somewhere, an egg "small as the dot of an i" is waiting.
The mood shifts from bareness and loss to quiet, curious hope. The speaker sounds observant and a little wry, and the poem ends on "open sesame," treating spring as a magic word that will unlock the emptied house again.