Answer:
Lucilia sp. ("green bottle fly"), a genus of blow flies in the family Calliphoridae
This was a bit of a challenging case. When it comes to bot flies, the overall body shape and, in particular, features of the spiracular plates, are used for identifying the genus of the infesting fly. In this case, the peritreme of the spiracular plates is complete, the button is clear and the slits are straight (below), which is consistent the with
Lucilia species.
Some readers suggested that this could be
Cochliomyia hominivorax, the New World screw-worm. However, the peritreme of this fly is incomplete and the button is not clear as what is seen in this case.
From Blaine Mathison (inspired by Kenny Rogers!)
You picked a fine time to eat
me, Lucilia
Diabetes mellitus and a lesion
on my heel
I’ve had some bad times
Infested by blow flies
But this time the lesion won’t
heal
You picked a fine time to eat
me, Lucilia