Wyjścia 8:17 Komentarz: Rashi, Rashbam, Sforno & Or HaChaim

משליח בך means I WILL INCITE AGAINST THEE. Another example is. (Deuteronomy 32:24) “I will incite (אשלח) the tooth of beasts against them”, where it signifies letting loose an animal against a person; old French inciter. English to incite.

והערוב, I believe that the Torah describes different kinds of wolves that are known by the word ,ערוב seeing that it is their habit to hunt at night as we know from Jeremiah 5,6. as well as from Tzefaniah 3,3. The fact that the wolf sets out when it gets dark, in the evening, and does not return until morning, is why this animal is referred to as ערוב, describing its habits.

וגם האדמה אשר הם עליה, a reference to the soil on which the houses stood which would become full of snakes and other creepers of the kind that inhabit the lower levels of the earth beneath its surface. The plague would result in the Egyptians feeling insecure in their homes even when they had shut every exit tightly.

את הערב this means all kinds of wild beasts, serpents, and scorpions (a mixture [ערב] of noxious creatures) in a crowd (Exodus Rabbah 11:3) — these played havoc with them. There is a reason given in the Midrash in the case of each plague why this particular one and why that: God came against them with the tactics of warlike operations as carried out by kings in orderly sequence: a government (monarch) who is besieging a city first destroys its water supply, then they blow the trumpets and sound an alarm in order to terrify and dismay them — thus [their water supply was turned into blood], the frogs croaked and made a noise etc. - just as you will find it stated in the Midrash of Rabbi Tanchuma 2:3:4.