What 's the difference between a gnat and a rodent?

"Middle English, from Old English gnĉtt; akin to Old English gnagan to gnaw first known use: before 12th century"

Do gnats have teeth?

"The gnatte is a litil fflye, and hatte culex..he soukeŝ blood and haŝ in his mouŝ a pipe, as hit were a pricke..And is a-countid a-mong volatiles..and greueŝ slepinge men wiŝ noyse & wiŝ bytinge and wakeŝ hem of here reste. [John of Trevisa, transl. of Bartholomew de Glanville's "De proprietatibus rerum," 1398]"