What is the definition of "Sheretz ha'Of
Rashi: It means very small (flying) creatures that creep along the ground, such as flies, hornets, gnats and locusts. 1
Ramban: It refers to flying creatures that walk on four legs. 2
See Ramban's objection to this explanation.
Which are called "Sheretz" because, like Sheratzim, their necks and heads are low, in line with their bodies - as opposed to birds that walk on two legs, whose necks are vertical and whose heads are high above their bodies. They move mainly via their wings and are constantly in flight, which is why they are called "Of Kanaf" or simply "Of" (Ramban).
Perhaps Rebbi discusses only locusts. However, he says "Kol Sheretz ha'Of"! (PF)
Why does the Torah add "asher lo Arba Raglayim"?
Ramban #1: Refer to 11:20:1:2*.
Ramban #2 (citing Toras Kohanim): To teach us that if they have five-legs, they are Kosher. 1
QUESTIONS ON RASHI
Rashi writes that "Sheretz ha'Of" are small, short creatures that creep along the ground. Some locusts (Sheretz ha'Of) are bigger than bats (which are called Stam Of)!
Mizrachi: In each category, the smallest are called Sheratzim 1 - among birds, flies; among insects, ants; among creatures, (the eight Sheratzim, i.e.) the weasel, mouse
If locusts are Sheretz ha'Of, all the more so bats should be! Perhaps only Tahor locusts are bigger than bats (PF); they are not included in "Sheretz ha'Of."