Within the list of kosher and non-kosher animals, there are several questions about the classification of the various animals. One, 11:19 records bats among the birds that cannot be eaten, but bats are mammals. Two, 11:20 records that insects that have four legs cannot be eaten, but all insects have six legs.
These questions are because today we are conditioned to classify animals based on the scientific zoological definitions and then bats do not belong with birds. However, as argued by Mary Douglas (1992) the Torah is classifying animals based on the different spheres, land, water and air, and the movement of the animals in these spheres. The classification is animals that move on land, animals that move in the water and animals that move in the air. Thus, the criteria that determine the classification of the animals are based on their means of movement. The legs of the insects are mentioned since this relates to their movement. Land animals can be eaten if they walk on cloven hoofs (and chew their cud) but if they walk on their paws, 11:27 or if they crawl on the ground, 11:42, then they are impure,
tamei, and they cannot be eaten. Fins are mentioned as the criteria for fish since the fins are the way that fish move. Whales and dolphins cannot be eaten since they move in the water, but they lack scales and fins, which is the criteria for kosher animals that move in the water. Similarly, bats are listed with animals that move in the air since they move in the air, even though they are not birds.
This understanding can explain why the Torah classified insects as having four legs. S. R. Hirsch (on 11:20, 1989, p. 278) explain that four legs means "that they have more than two feet and this multiplicity of feet is just the characteristic difference between a bird and
sherets bird (insect)." I would follow this latter idea that the four legs by insects is to separate them from birds. Birds are classified based on their movement through air even though they walk on the ground since they only walk on two legs. If they would walk on four legs, then they would have been judged as land animals. Thus, by insects the Torah uses the term four legs since once they have four legs then they are judged by their movement on land and not in the air and then they fail the criteria of land animals (chew their cuds and have split hoofs).
Why did the Torah classify the animals based on their locomotion in the different spheres? Douglas argues that this classification reflects society’s values. This is possible, but maybe this method was a relatively easy way for people to classify the animals without requiring a degree in zoology.
Bibliography:
Douglas, Mary, 1992 (first published in 1966), Purity and Danger, London: Rutledge.
Hirsch, S. R. (1808-1888), 1989, The Pentateuch, rendered into English by Isaac Levy, second edition, Gateshead: Judaica Press.