Swarms of flies is an unfortunate translation of a single substantive in the singular number, accompanied by the article. A mixture, etc; is nearly as bad. The writer must mean some one definite species of animal, which he called "the 'arob."
On the probable identification of the animal, see the Introductory paragraph to this Chapter. And also the ground. The 'arob, like the frogs, was to plague them both inside their houses and outside, but especially inside.