That is not likely to be true. Even in the case of the use of isolated or crude preparations of viper VENOM, taking it orally would do nothing whatsoever (barring entry into the bloodstream via injuries to the oral, oesophageal mucosa, etc.
Snake venom, along with that of scorpions, spiders (although there are a few spider and wasp venoms that are based on small molecules rather than proteins), ants, etc. the venom components that do the main damage are almost entirely, almost always proteinaceous in nature, or peptides (smaller than proteins, chains or cyclic constructs of aminoacids) and MUST be injected into the system to be active. One could, if there were no injuries to permit its access to the system, swallow snake venom to no ill effect, being a protein/peptide based concoction these venoms will simply be torn to pieces by the digestive system, no different than a piece of meat.
And thus can produce no immunizing effect. Ergo, the roman claim is false. IIRC the theriacs often allayed symptoms of illness due to the large component being opium-based. But certainly would not prevent death due to a bite from a viper/elapid snake envenomation.
Ants? think fire ants are bad news, you lot should have seen the fucking bastards I saw whilst trekking through some dense turkish forests on holiday. Big buggers, average what I took to be workers from behaviour, coming out of an undisturbed nest were approximately, on average, a centimeter and a quarter to 1.5cm long, with HUGE heads and mandibles to match. In build, they were heavy, squat, resembling african driver ants in all but size and the lack of the driver/army ant behaviour of marching in columns. I didn't get stung or bitten, I don't know if the species concerned did sting but I sure as blue buggery wouldn't have wanted to find out (not after the vicious sting I got from some species of tropical nettle, that was bad enough. Felt like somebody whacked my arm with a hammer, after being stung on the fingers.
Black widows ey......well we VERY seldom get them in the UK, they aren't native, but have sort-of invaded to a degree and the odd one pops up.
I had a brown widow as a pet, and yes, I have been bitten. The effects were NOT pleasant. Severe cramping all over, was the main effect, felt like I got worked over with a baseball bat by a gorilla. Although the bite was my fault, pissing her off changing stuff in the enclosure, adding crickets and the like whilst she was guarding two egg sacks.
Unfortunately, the fucking pigs brought down one of their abusive, fruitless harassment raids on me at the time, and murdered both my pet, her babies, hundreds of newly hatched brown widow spiderlings and the second, unhatched egg sack, all killed, and for no reason whatsoever, just senseless, mindless carnage.
Anybody where they occur naturally, (any of the latros) be willing maybe, to catch me one, or more, for a fair price?
ponds from now on.