The “flies” of the Bible included the common housefly, as well as other two-winged insects. Many of these were biting insects. This explains the “devouring” flies of (Psalm 78:45). The flies visited as a plague upon the Egyptians probably included the housefly and the stinging sand fly, as well as gnats and mosquitoes.

The prophet Isaiah’s reference to the “fly that is in the farthest part of the rivers of Egypt” (Isa 7:18) may have been a symbol of swarms of Egyptian soldiers. Or, he could have had in mind the dreaded tsetse fly of Africa, which spreads sleeping sickness. Still another possibility is the olive fly, which could ruin a crop of ripe olives.

Solomon’s “fly in the ointment” (Eccl. 10:1) has become a proverb. So also has Jesus’ “straining out a gnat”– which referred to the custom of straining wine to take out the impurities before it was served (Matt. 23:24).

Source: [Anon-Animals]