Hullin, Chapter Three, Mishnah Seven
Some types of locusts are clean and can be eaten (see Leviticus 11:20-25). Our mishnah teaches the characteristics of locusts and fish that can be eaten.
1) Of locusts: all that have four legs, four wings, leaping legs, and wings covering the greater part of the body, [are clean.]
a) Rabbi Yose says: its name must be locust.
2) Of fishes: all that have fins and scales [are clean].
a) Rabbi Judah says: there must be [at least] two scales and one fin.
3) The scales are those which are immovable, the fins are those [wings] by which it swims.
Section one: For a locust to be clean it must have four legs and wings. It must also have two legs above its other legs with which to leap (see Leviticus 11:21). And its wings must cover the greater part of its body.
Rabbi Yose adds that it must be called a locust. If it has all of the signs of being kosher but it is not called a locust then it is not kosher. We can see here just how important a name can be to the status of the validity of the animal.
Section two: The Torah states this rule explicitly, see Leviticus 11:9. Rabbi Judah adds that there must be at least two scales.
Section three: The mishnah now defines scales and fins. Scales are those that are immovable, meaning that the fish does not have control over them. Whereas the fins are like wings to a bird in that the fish swims by moving his fins.