A more cheerful fly--also in unrhymed tercets, coincidentally:

My Fly
by C. K. Williams

for Erving Goffman, 1922-1982

One of those great, garishly emerald flies that always look freshly generated from fresh excrement
and who maneuver through our airspace with a deft intentionality that makes them seem to think,
materializes just above my desk, then vanishes, his dense, abrasive buzz sucked in after him.

I wait, imagine him, hidden somewhere, waiting, too, then think, who knows why, of you--
don't laugh--that he's a messenger from you, or that you yourself (you'd howl at this),
ten years afterwards have let yourself be incarnated as this pestering anti-angel.

Now he, or you, abruptly reappears, with a weightless pounce alighting near my hand.
I lean down close, and though he has to sense my looming presence, he patiently attends,
as though my study of him had become an element of his own observations--maybe it is you!

Joy! To be together, even for a time! Yes, tilt your fuselage, turn it towards the light,
aim the thousand lenses of your eyes back up at me; how I've missed the layers of your attention,
how often been bereft without your gift for sniffing out pretentiousness and moral sham.

Why would you come back, though? Was that other radiance not intricate enough to parse?
Did you find yourself in some monotonous century hovering down the tidy queue of creatures
waiting to experience again the eternally unlikely bliss of being matter and extension?

You lift, you land--you're rushed, I know; the interval in all our terminals is much too short.
Now you hurl against the window, skid and jitter on the pane: I open it and step aside
and follow for one final moment of felicity your brilliant ardent atom swerving through.

[p. 289, Americans' Favorite Poems: The Favorite Poem Project Anthology, edited by Robert Pinsky and Maggie Dietz]

Oh, and I forgot "I heard a fly buzz - when I died - (591) by Emily Dickinson, here. The legalistic emphasis of S3 reminds me of the Dickinson spider poem that Roger posted above. Yet again, there's no legal recourse against the beastie, although the implications seem far more sinister here.