Gemara

51b (Rava) Question: (A tradition obligates one who eats a Biryah (a creation) of any size.) If one ate an incomplete ant, what is the law?

Does the tradition apply only to a complete creation?

Or, does it apply to an amount that could live?

Answer (Rav Yehudah of Diskarta - Beraisa) Suggestion: "(One who touches) Bahem (becomes Tamei)" - perhaps only a complete Sheretz (rodent) is Metamei!

Rejection: "(If will fall) from them (into a oven, everything inside becomes Tamei)."

Suggestion: Perhaps even part of one is Metamei!

Rejection: It says "Bahem"!

Resolution: One is Tamei if he touches an amount equal to a full Sheretz. Chachamim estimated that this is the size of a lentil, which is the size of a Chomet (snail or lizard) when it is created.

Summation of answer: We require a full creation (the Shi'ur of a full Chomet, even though surely it could live even if a small amount was cut off).

Rejection (Rav Shemayah): Less than the size of a lentil is not important for Tum'ah, for a Nefesh never enters a Sheretz of this size. Something that was alive is important, even if it is missing something not vital to life!

Chulin 96b (Mishnah): If the Gid ha'Nasheh was cooked with permitted Gidim, and we cannot recognize the Gid ha'Nasheh, they are all forbidden.

99b - Question: It should be Batel in the majority!

Answer: It is a Biryah, so it is never Batel.

102b (Rav): If one eats a (whole) live Tahor bird he is liable, no matter how small it is. If it was dead (Neveilah), he is liable only for a k'Zayis (the volume of an olive).

One is liable for a Tamei bird, whether alive or dead, no matter how small it is.

Makos 13a (Mishnah - R. Shimon): One who eats any amount of Tevel is lashed;

Chachamim say, he is lashed only if he eats a k'Zayis (an olive's worth).

R. Shimon: Don't you agree that one is liable for eating a (full) ant of any size?

Chachamim: That is because it is a full creation.

R. Shimon: A wheat kernel is also a full creation!

16b (Rabah bar Rav Huna): If one ground up nine ants and completed the quantity of a k'Zayis with an intact ant, he is lashed five times for eating a full creation, and once for eating a k'Zayis of Neveilah.

Rishonim

The Rif and Rosh (Chulin 35b and 7:33) bring the Mishnah and Gemara in Chulin. (Some texts delete this from the text of the Rif.)

Rosh: Gid ha'Nasheh, Ever Min ha'Chai, and Tamei fish and birds are Briyos. Rashi says that a Tahor bird that became Neveilah is not, for it was not a Neveilah when it was formed. This is difficult. Chelev on the stomach or of the kidneys, Orlah and Kil'ai ha'Kerem were forbidden from the beginning, but they can be Batel! Rather, we require that if it were divided, it would not be called by its name. 'Bird' connotes a full bird. If it is incomplete, one calls it a piece. The same applies to Gid ha'Nasheh and Ever Min ha'Chai. However, even a piece of Neveilah is called Neveilah. Regarding Shor ha'Niskal (an ox sentenced to be stoned) it says "Lo Yochal Es Besaro"; one may not eat even part (the Isur does not depend on the full Biryah.) Regarding Sheratzim it says "do not eat them", which connotes when they are complete. One is liable for transgressing a Shevu'ah to eat any amount 'because it is like a Biryah' (Shevuos 21b). I.e. one is liable for a Biryah because it is as if the Torah specified a Biryah of any size.

Question (Rosh): Ever Min ha'Chai and Gid ha'Nasheh do not have a Neshamah!

Answer (Rosh): Chachamim answered R. Shimon according to his reasoning. We would not obligate for a wheat kernel even if it were a Neshamah, for the Torah forbids Tevel, not wheat. Even if you disagree, you should admit that it is unlike an ant! Some say that even Chachamim require that a Biryah come from something with a Neshamah.

Ran (36a DH Garsinan): There are three conditions to be considered a Biryah:

It itself must be forbidden, i.e. from its initial creation, such as Gid ha'Nasheh. A Tahor bird that became a Neveilah is not considered a Biryah, because the Isur is not due to being a Biryah. This is why one is liable for a dead Tahor bird only for a k'Zayis. We suggested (Zevachim 73a) that a Shor ha'Niskal should be Batel, and answered that living animals are not Batel. It was not considered a Biryah because the Isur was external.

It must be Briyas Neshamah. R. Shimon lashes one who eats a wheat kernel, because it is a Biryah. Chachamim exempt, because it has no Neshamah. Even though Chelev has a Neshamah, it is not a Biryah because it is not all in one place.

It must be complete. If one limb was mashed, it is not a Biryah, even if the limb was not vital. One who ate mashed ants is liable for a Biryah only if one was intact. The Gemara in Nazir never resolved whether or not one is lashed in such a case. Therefore, we are lenient about mixtures, for mid'Rabanan a Biryah is not Batel, and we are lenient about a Safek mid'Rabanan.

Rambam (Hilchos Ma'achalos Asuros 2:22): If an ant is missing even one of its legs, one who eats it is lashed only if he eats a full k'Zayis. Therefore, one who eats a complete fly or mosquito is lashed.

Poskim

Shulchan Aruch (YD 100:1): A Biryah, such as an ant, Tamei bird, Gid ha'Nasheh, Ever Min ha'Chai or egg witha chick inside, or similar things, is not Batel even in 1000. It is a Biryah only if it was alive. This excludes a wheat(kernel) of Isur. Also, this is only if it was forbidden from its initial creation. This excludes a Tahor bird that became a Neveilah, or an ox that must be stoned (for killing a person). It must be a complete matter, that if it was divided, it would not be called by its name. This excludes Chelev. Also, it must be complete.

Rema: The essence of the Gid ha'Nasheh is only on the Kaf (the round end of the thigh). It is about four fingers wide. If this is complete, it is called a Biryah.