Read "Plague from the Abatoir" at The Eldritch Dark:
There's an unmistakable note of frustration in these lines from Clark Ashton Smith (CAS), as evidenced by the tone of the verbs: "Fall", "kill", "buzz", "blunder". Not a lot of happy occurrences going on here!
Anyone who has ever encountered blow flies will share CAS' sentiments, as these loud and persistent creatures use their acute sense of smell to locate carrion. Science tells us that they can detect a fresh kill up to a mile away, so it's no wonder they created a "Plague" for CAS given his residence not far from a slaughter-house in Auburn, California.