swarms of flies. Ellipsis supplied. But should not be thus limited. Hebrew root shows that they were mixed: all sorts of insects.
and. Note the Figure of speech Polysyndeton (App-6) emphasises the universality of the plague.
Related Commentaries of Exodus 8:21
Exodus 8:21
21 Else, if thou wilt not let my people go, behold, I will send swarmsd of flies upon thee, and upon thy servants, and upon thy people, and into thy houses: and the houses of the Egyptians shall be full of swarms of flies, and also the ground whereon they are.