The speaker is driving beside a river and keeps having to clear mayflies off the windshield, each one a small smear, an "inkblot" of an anonymous death. Instead of just being annoyed, he starts imagining the insect's strange life: a long stretch of waiting in an unfinished state, then a single day of sudden change, with wings, air to breathe, one chance to mate, and no mouth left to eat with.
The haiku at the end turns this toward the speaker's own life. It's spring again, and he is scheduling his first Covid shot. The poem never says outright why the mayflies matter to him, but the link seems clear enough: after a long period of holding still and waiting, he too is on the edge of a release. The mood is quiet, a little wry, and thoughtful. There's sadness in the small deaths on the glass, but also a hopeful sense of renewal as the season turns.