Laudator Temporis Acti

Pausanias 5.14.1 (tr. J.G. Frazer):

They say that when Hercules, the son of Alcmena, was sacrificing in Olympia, he was greatly plagued by the flies; so either out of his own head or by the advice of some one else, he sacrificed to Zeus Averter of Flies, and thus the flies were sent packing across the Alpheus. In the same way the Eleans are said to sacrifice to Zeus Averter of Flies at the time when they drive the flies out of Olympia.

φασὶ δὲ Ἡρακλεῖ τῷ Ἀλκμήνης θύοντι ἐν Ὀλυμπίᾳ δι’ ὄχλου μάλιστα γενέσθαι τὰς μυίας· ἐξευρόντα οὖν αὐτὸν ἢ καὶ ὑπ’ ἄλλου διδαχθέντα Ἀπομυίῳ θῦσαι Διί, καὶ οὕτως ἀποτραπῆναι τὰς μυίας πέραν τοῦ Ἀλφειοῦ. λέγονται δὲ κατὰ ταὐτὰ καὶ Ἠλεῖοι θύειν τῷ Ἀπομυίῳ Διί, ἐξελαύνοντες τῆς Ἠλείας Ὀλυμπίας τὰς μυίας.

Frazer in his commentary ad loc. (words in square brackets added by me):

This sacrifice is mentioned also by Clement of Alexandria (Protrept. ii.38, p. 33, ed. Potter). Pliny says: "The Eleans invoke the flycatching god [Myiacoren deum muscarum], because the swarms of flies breed pestilence; and no sooner is the sacrifice offered to the god than the flies perish" (Nat. Hist. x.75). Aelian affirms that during the Olympic festival the flies voluntarily retired to the opposite bank of the Alpheus and swarmed back when the festival was over (Nat. An. v.17). Pausanias tells us (viii.26.7) that at Aliphera in Arcadia the festival of Athena was opened with sacrifice and prayer to the Fly-catcher [Μυιάγρῳ]; and that after the sacrifice the flies gave no more trouble. At the festival of Apollo in the island of Leucas an ox was sacrificed to the flies, which, glutted with the blood, thereupon disappeared. Aelian, who reports this (Nat. An. xi.8), adds that the flies of Pisa (meaning Olympia) were more virtuous, because they did their duty, not for a consideration, but out of pure regard for the god. At the shrine of Hercules in the Ox-market at Rome flies were excluded, because when Hercules was handing the flesh to the priests he had prayed to the fly-catching god [Myiagrum deum] (Solinus, i.11)....As for the flies at Olympia, the hot climate and the low damp situation still breed them in multitudes. I never saw such swarms of flies anywhere. In the house where I stayed they were a plague. If I had thought that a sacrifice to Zeus Averter of Flies would have rid me of them, I would gladly have offered it.

I too would be tempted to sacrifice to Zeus Apomyios, if I thought he could get rid of mosquitoes and deer ticks. As it is, when I'm working outside in the summer, I wear a "bug jacket" made of mosquito netting, which also encloses my head, and I tuck the cuffs of my pants into my socks.

See also Joseph William Hewitt, "The Propitiation of Zeus," Harvard Studies in Classical Philology 19 (1908) 61-120 (at 115-116), and Christopher Michael McDonough, "Forbidden to Enter the Ara Maxima: Dogs and Flies, or Dogflies?" Mnemosyne 52 (1999) 464-477.