All the swarming things that swarm on the ground you shall not eat… (Leviticus 11:42)
Continuing the ban on eating swarming things, this next phrase includes a prohibition against eating insects that grow in fruit or grain (see Sifra on parshas Shemini). Such insects are technically permitted if they never left the fruit, but if they ever left the fruit and returned to it, they are prohibited. (Chances are the insects in your vegetables are not wearing tiny house arrest-style ankle monitors to alert you to their whereabouts.)
This law also applies to the worms inside of fish. Those in the fish’s intestines are prohibited because they may have come from outside but those in the flesh (or between the flesh and the skin) are permitted because they never emerged (and are considered as if to have “grown” from the fish). Similar laws apply to parasites found in animals; the Talmud in Chulin (67b) applies the verse “you shall find their carcasses detestable” to include the worms in an animal, which are not rendered permissible through the animal’s slaughter.
Regarding eating a creeping land animal, we said that while eating any size amount is prohibited, one would only be liable to punishment for eating an olive-sized piece. These creatures are different because they can be really tiny. A whole creature cannot be nullified in a mixture even if it’s outnumbered 1,000 to one by permissible food items. Accordingly, one is liable for eating such a bug no matter how small (see Talmud Makkos 13a). (If it’s not whole, then the usual size limits apply.)
An interesting side note: the midpoint of the Torah in terms of letters occurs in this verse, but we are only slightly past the one-quarter mark in terms of mitzvos. (The earlier books contain much more narrative and less legal material than the later books.)
This mitzvah applies to both men and women in all times and places. It is discussed in the Talmud in tractate Chulin on pages 67a-b and in Makkos on 13a and 16b. It is codified in the Shulchan Aruch in Yoreh Deah 83 and 84. It is #178 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #98 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.