^a Gk. ἐρπετόν. Ἐρπετόν includes invertebrates and small, "crawling" vertebrates [but not bats by virtue of being grouped separately, along with birds, in 11:19]. "Flying" (and even more broadly "winged") narrows ἐρπετόν down to insects. The only winged invertebrates, flying or not, are insects.

^b The insects being identified here as walking on four feet are additionally identifed with another two legs for leaping in the next verse, i.e., six legs total. Winged insects that walk on four feet include locusts—which jump with their hind legs and walk on their mid- and forelegs—and mantises and Nymphalidae butterflies—which conversely walk on their mid- and hind legs, not their forelegs.

^c There are no four-footed invertebrates; the only winged invertebrates are insects, which have six legs. Key to not misconstruing "walking on four feet" with having only four feet, is that this verse does not stand alone, separate from 11:21 and 11:22. This verse should not be read as expressly prohibiting only flying insects that walk on four feet, and by omission no others; all invertebrates are prohibited, regardless of wings and legs.

^d Leviticus doesn't explicitly prohibit the eating of flying insects, but Dt. 14:19 does: All winged [flying] animals that creep are unclean to you; ye shall not eat of them [Brenton]. (Non-flying invertebrates and small, "crawling" vertebrates are prohibited in Lv. 11:29.) Lv. 11:20-22 defines an exception for four types of locust to the general prohibition in Deuteronomy: within the prohibition against all flying invertebrates, there is a subset of flying, six-legged insects that walk on four feet [which incl. mantises and Nymphalidae butterflies], which is still prohibited as a whole; but within this subset is another subset that have an additional pair of legs for leaping [which excl. mantises and Nymphalidae butterflies]; within this subset are four types of locusts that are permitted.