Exodus 8:17

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. 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.

כִּ֣י אִם־אֵינְךָ֮ מְשַׁלֵּ֣חַ אֶת־עַמִּי֒ הִנְנִי֩ מַשְׁלִ֨יחַ בְּךָ֜ וּבַעֲבָדֶ֧יךָ וּֽבְעַמְּךָ֛ וּבְבָתֶּ֖יךָ אֶת־הֶעָרֹ֑ב וּמָ֨לְא֜וּ בָּתֵּ֤י מִצְרַ֙יִם֙ אֶת־הֶ֣עָרֹ֔ב וְגַ֥ם הָאֲדָמָ֖ה אֲשֶׁר־הֵ֥ם עָלֶֽיהָ׃

Else, if thou wilt not let my people go, behold, I will send swarms of gnats upon thee, and upon thy servants, and upon thy people, and into thy houses: and the houses of Miżrayim shall be full of swarms of gnats, and also the ground on which they are.