Parah, Chapter 9, Mishnah 2. This chapter takes up the many ways in which the sanctified water, the water into which the ashes of the parah adumah have been placed, can be ruined and rendered unfit for sprinkling. Here the Mishnah pictures a flask of that water standing ready for use, and something small and living drops inside.
The Mishnah begins: "Naflu lesocho shekatzim uremasim venivka'u, o shenishtanu mareihen, pesulin" - should crawling creatures or vermin drop into the flask and split apart there, or should the water take on a different appearance, it may no longer be used for sprinkling.
Two separate problems are described. The first is that the creature came apart inside the flask, so that its own moisture seeped out and mingled with the sanctified water. The second does not even require that the creature break open: if the appearance of the water changed on account of what fell in, that alone disqualifies it.
The beetle
The Mishnah continues: "Chipushis, bein kach uvein kach pesulah, mipnei shehi kishfoferes" - a beetle disqualifies the water either way, because it is like a tube.
With a beetle it makes no difference whether it broke apart and no difference whether the color of the water stayed exactly as it was. The beetle is built like a hollow pipe. The water enters it, meets the moisture inside it, and then trickles back out the other end. Since the water passes straight through the creature and picks up its moisture on the way, the flask is unfit in every case.
The tiny insects of the grain
The Mishnah then brings an opposing opinion: "Rabbi Shimon veRabbi Eliezer ben Yaakov omrim: hadirah vehakinah shebatevuah, kesheirim, mipnei she'ein bahen leichah" - in the view of these two Sages, the dirah and the kinah, the creatures found among stored grain, leave the water usable, since no liquid whatsoever is present in them.
The reference is to the barely visible bugs that breed in grain kept in storage. These Tannaim hold that even should such a bug tumble into the flask and come apart inside it, nothing at all was given off into the water, for the creature has no fluid in it to give off, and the water may still be sprinkled.
The halachah, however, does not follow Rabbi Shimon and Rabbi Eliezer ben Yaakov on this point.
The lesson of the mishnah is the extraordinary delicacy of the mei chatas. A creature small enough to be overlooked in a sack of grain, a beetle no one would think twice about, is enough to end the usefulness of water that was prepared with such care.