What is the definition of "Sheretz ha'Of? ha'Holech al Arba"?
Rashi: Very small (flying) creatures that creep along the ground, such as flies, hornets, gnats and locusts. 1
Ramban: Flying creatures that walk on four legs. 2
RS"R Hirsh: 'Four' means more than two. Really, all of them have at least six legs. Rebbi in the Sifra (3:5:10), is Machshir it if it has [at least] five. 3
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).
This requires investigation however. Perhaps Rebbi is discussing only locusts. However, he says "Kol Sheretz ha'Of"! (PF)
QUESTIONS ON RASHI
Rashi writes that "Sheretz ha'Of" are small, short creatures that creep along the ground. But some locusts (Sheretz ha'Of) are larger than bats (which are called S'tam Of)?
Mizrachi: In each category, the smallest are called Sheratzim 1 - among birds, flies; among insects, ants; among creatures, (the eight Sheratzim) the weasel, the mouse?
Refer to 11:20:1:2 and the note there.
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."
