These are unclean to you of all that creep,.... Unfit for food, and not to be touched, at least when dead, as in the next clause, that is, these eight sorts of creeping things before mentioned, as the Targum of Jonathan expresses it, and these only, as Maimonides says (r):
whosoever doth touch them when they are dead shall be unclean until the even; for touching them while alive did not defile, only when dead; and this the Jews interpret, while they are in the case in which they died, that is, while they are moist; for, as Ben Gersom says, if they are so dry, as that they cannot return to their moisture, they do not defile; for which reason, neither the bones, nor nails, nor nerves, nor skin of these creeping things, defile; but, they say (s), while the back bone is whole, and the bones cleave to it, then a creeping thing is reckoned moist, and while it is so it defiles. (r) Hilchot, Abot Hatumaot, c. 4. sect. 14. (s) Maimon. & Bartenora in Misn. Niddah, c. 7. sect. 1.

whosoever doth touch them, when . . . dead, shall be unclean until the even--These regulations must have often caused annoyance by suddenly requiring the exclusion of people from society, as well as the ordinances of religion. Nevertheless they were extremely useful and salutary, especially as enforcing attention to cleanliness. This is a matter of essential importance in the East, where venomous reptiles often creep into houses and are found lurking in boxes, vessels, or holes in the wall; and the carcass of one of them, or a dead mouse, mole, lizard, or other unclean animal, might be inadvertently touched by the hand, or fall on clothes, skin bottles, or any article of common domestic use. By connecting, therefore, the touch of such creatures with ceremonial defilement, which required immediately to be removed, an effectual means was taken to prevent the bad effects of venom and all unclean or noxious matter.

The words, "these are unclean to you among all swarming creatures," are neither to be understood as meaning, that the eight species mentioned were the only swarming animals that were unclean and not allowed to be eaten, nor that they possessed and communicated a larger amount of uncleanness; but when taken in connection with the instructions which follow, they can only mean, that such animals would even defile domestic utensils, clothes, etc., if they fell down dead upon them. Not that they were more unclean than others, since all the unclean animals would defile not only persons, but even the clothes of those who carried their dead bodies (Leviticus 11:25, Leviticus 11:28); but there was more fear in their case than in that of others, of their falling dead upon objects in common use, and therefore domestic utensils, clothes, and so forth, could be much more easily defiled by them than by the larger quadrupeds, by water animals, or by birds. "When they be dead," lit., "in their dying;" i.e., not only if they were already dead, but if they died at the time when they fell upon any object.