The two following Hebrew terms denote flies of some kind:
+ 'Arob ("swarms of flies," "divers sorts of flies," Authorized Version), the name of the insect or insects which God sent to punish Pharaoh; see (Exodus 8:21-31; Psalms 78:45; 105:31) The question as to what particular species is denoted, or whether any one species is to be understood, has long been a matter of dispute. As the arob are said to have filled the houses of the Egyptians, it seems not improbable that common flies (Muscidae) are more especially intended. The arob may include various species of Culicidae (gnats), such as the mosquito; but the common flies are to this day in Egypt regarded as a "plague," and are the great instrument of spreading the well-known ophthalmia, which is conveyed from one individual to another by these dreadful pests. "It is now generally supposed that the dog-fly is meant, which at certain seasons is described as a far worse plague than mosquitos. The bite is exceedingly sharp and painful, causing severe inflammation, especially in the eyelids. Coming in immense swarms, they cover all objects in black and loathsome masses, and attack every exposed part of a traveller's person with incredible pertinacity."--Cook.