Halakhah su Levitico 11:31

The laws of the commandment: For example, that which they, may their memory be blessed, said (Mishnah Kelim 1:1) that eight swarming creatures are called a primary source (av) of impurity, meaning to say that they render a man and vessels impure by touch; and if the vessels are of clay, they render them impure once they enter its space (hollow). And this is the distinction that there is in the matter between that which is called a primary source and that which is called a derivative (velad) - as that which is called a primary source renders a man and vessels impure, but that which is called a derivative does not render a man and vessels impure. And the swarming creatures only render impure after they die, as it is stated (Leviticus 11:31), "in their being dead, he shall become impure." And it is specifically these swarming creatures that render impure, but all other swarming creatures - the snake, the scorpion and all that are similar to them - do not render impure at all. And the law of the swarming creatures is that they do not render impure by carrying without touching [directly]; and that one that touches them does not render his clothes impure when they are upon him at the time that he touches them. And the measure of the impurity of the swarming creature is like [the size of] a lentil. As behold, we found that the Torah renders them pure undifferentiatedly, and there are some of these swarming creatures that are only like a lentil. And all of the swarming creatures combine to [the measure of] like a lentil - meaning to say, even a little of this one and a little of that one combine to [form the size of] a lentil, to render impure; such that you not say that the liability is only like a lentil from one [creature] by itself (Meilah 15b).