Comentario sobre Levítico 11:21

על ארבע means UPON FOUR legs.

אשר לו כרעים. which have jointed legs above their feet, etc. In my treatise on Tur Yore Deyah 85 I have explained in detail that Rashi is correct that these winged creatures have to have legs which are attached close to their throats. People who permit these creatures to be eaten because there are no species which conform to this description are in error. The fact is that of the four, respectively eight categories of winged creatures listed in verses 22 as permitted to eat, only one category is found in our parts of the world. The eight kinds of forbidden swarming things listed in verses 29 and 30 also do not have their habitat in parts of the globe inhabited by Jews. Most of the winged things that do abound in our part of the globe do not have the kind of jointed legs that would make them edible according to the Torah's criteria. Most of these details are discussed in Chulin 63-65. The important thing is that the fact that we do not find the species mentioned in the Torah in our parts of the globe is no proof that they do not exist and that our sages have misinterpreted the Torah. Any G'd-fearing Jew will abstain therefore from eating any of these things and will protest when he sees other Jews eating them. For the last 12 years after I have published these words and people stopped eating these kinds of "grasshoppers," our land has not been afflicted by the plague of locusts. This is proof that observance of Torah and the commission of good deeds protects us against natural disasters.

לנתר, to jump, to shake, as per Onkelos. The root occurs in this sense in Chabakuk 3,6 ויתר גוים, “He makes the earth shake.” This transitive mode of the root נתר, natar, also occurs in Genesis 2,21 from the root נפל, nafal.

אשר לו כרעים, “which have jointed legs,” The word לו is spelled לא in the text, to hint at the dual nature of these (additional) legs. It is read as if it had been spelled as shown.

ממעל לרגליו ABOVE ITS FEET — Quite close to its neck it has something like two feet, in addition to its real feet; when it wants to fly and to spring off the ground it presses itself strongly on the ground with those two knees and so it flies. There are many of these, such, as those which we call longouste (sea-locusts), but we are not expert in regard to them, — as to which we are not permitted as food, — because four characteristic marks of cleanness are mentioned by our Rabbis in respect to them, viz., four feet, four wings, and קרסולים, i. e. the כרעים mentioned here, and that their wings cover the greater part of them. All these characteristics are present in these which are found amongst us, but there are some of them which have a long head and there are some which have no tail, matters which do not affect the question of cleanness (Chullin 59a). It is, however, necessary in order that they shall belong to the clean species that they should bear the name חגב (Chullin 55b), and in this respect we do not know how to distinguish one from another [i. e. we have no tradition as regards to these locusts whether they belong to the species which was in olden (Talmudic) times called חגב or not, and consequently in spite of the other four marks of cleanness we cannot distinguish them from unclean locusts which also show the same marks but are unclean because they do not bear the name חגב].