Charles Ellicott Commentary Leviticus 11:21

"Yet these may ye eat of all winged creeping things that go upon all fours, which have legs above their feet, wherewith to leap upon the earth." — Leviticus 11:21 (ASV)

Of every flying creeping thing. —Rather, of all winged creeping things. Having laid down the general rule that those creatures that creep along on their feet like quadrupeds, and also have wings, must not be eaten, the Lawgiver now mentions those that form an exception.

Which have legs above their feet. —Better, which have knees above their hind legs, that is, those that have the third or hindmost pair of legs much longer and stronger than ordinary insects. Those insects, therefore, in whose hind legs the second joint is much larger and stronger, by which they are enabled to leap or raise themselves up with great force and leap a great distance on the earth, are excepted. These are the locusts.

The canonical law that was in effect during the Second Temple period defines in more detail the characteristics of clean locusts. We are told that a clean locust has:

1. four front feet,
2. four wings,
3. two springing feet, and
4. the wings so long and broad that they cover the greater part of the back of the insect's body.

If it possesses these four characteristics, it is clean, whether it has a tail or not, and whether it has an oblong or round head.