||If you do not let My people leave, I will send swarms of harmful creatures [to attack] you, your officials, your people, and your homes. The houses of Egypt, and even the ground upon which they stand, will be filled with these creatures.
Ki im-eynecha meshale'ach et-ami hineni mashli'ach becha uva'avadeycha uve'amcha uvevateycha et-he'arov umale'u batey Mitsrayim et-he'arov vegam ha'adamah asher-hem aleyha.
Arov in Hebrew. In the Midrash there is a dispute. Rabbi Nechemia says that arov denotes flies, and Rabbi Yehudah states that it denotes a mixture of wild animals (Sh'moth Rabbah 11:4). Most Midrashim accept the interpretation that arov is wild animals, and this opinion is reflected in most later commentaries (Targum Yonathan; Rashi; Ibn Ezra; Radak, Sherashim; Ibn Janach; Josephus, Antiquities 2:14:3). This would take the verse, 'He sent the arov and it ate them' (Psalms 78:11) in its most literal sense. However, even here, some say that the animals only ate their food (Ralbag).
Still, there are many sources that interpret arov as flies (cf. Haggadah, Minhag Teiman 42; Midrash Or HaAfelah, quoted in Torah Sh'lemah 65). Some ancient sources identify the arov as dog-flies (Septuagint; Ethiopic edition of Yov'loth 48:5), or blood-suckers (Philo, De Vita Mosis 2:101). Another source states that it is a mixture of insects and snakes (Sefer HaYashar). It is also possible that the Hebrew arov is related to the ancient Egyptian a'ov, denoting beetles, specifically the scarab or dung beetle.
Other sources identify the arov as an invasion of a specific kind of animal, either wolves (Rashbam), panthers (Midrash Tehillim 78:45), eagles or other birds (Ibid.), or even giant squid (silonith in Hebrew; Ibid.; Sefer HaYashar p. 207; Sekhel Tov; Midrash Aggadah; Midrash VaYosha; see The Torah Anthology 4:254, note 18). See Wisdom of Solomon 11:15-18.