The root word from which this term is drawn means to “swarm” or “teem.” (Ge 8:17; Ex 8:3) The noun appears to apply to small creatures to be found in large numbers. (Ex 8:3; Ps 105:30; compare Ex 1:7.) It first occurs at Genesis 1:20 with the initial appearance of living souls on the fifth creative day when the waters began to swarm with living souls. The Flood destroyed earthly ‘swarming things’ outside the ark.—Ge 7:21.
The law regarding clean and unclean things shows that the term may apply to aquatic creatures (Le 11:10); winged creatures, including bats and insects (Le 11:19-23; De 14:19); land creatures, including rodents, lizards, chameleons (Le 11:29-31); as well as creatures traveling on their “belly” and multilegged creatures (Le 11:41-44). Many, but not all, of these were “unclean” as food under the Law.