There used to be a theory that ants had passwords. Ants live in colonies and they don’t let in strange ants from other colonies. This raises the question of how they know who’s who. The password theory was a bit odd, but it was reasonably popular among whimsical Victorian naturalists until it was thoroughly debunked by Sir John Lubbock, 1st Baron Avebury, following some experiments in the 1870s:

It has been suggested that the Ants of each nest have some sign of password by which they recognize one another. To test this I made some insensible. First I tried chloroform, but this was fatal to them; and… I did not consider the test satisfactory. I decided therefore to intoxicate them. This was less easy than I had expected. None of my Ants would voluntarily degrade themselves by getting drunk. However, I got over the difficulty by putting them into whisky for a few moments. I took fifty specimens, twenty-five from one nest and twenty-five from another, made them dead drunk, marked each with a spot of paint, and put them on a table close to where other Ants from one of the nests were feeding. The table was surrounded as usual with a moat of water to prevent them from straying. The Ants which were feeding soon noticed those which I had made drunk. They seemed quite astonished to find their comrades in such a disgraceful condition, and as much at a lost to know what to do with their drunkards as we were. After a while, however, to cut my story short, they carried them all away; the strangers they took to the edge of the moat and dropped into the water, while they bore their friends home into the nest, where by degrees they slept off the effects of the spirit. Thus it is evident that they know their friends even when incapable of giving any sign or password.

Source:

Forsyth, Mark. “Evolution.” A Short History of Drunkenness. Three Rivers Press, 2017. 10, 11. Print.

Further Reading:

https://en.m.wikipedia.org/wiki/John_Lubbock,_1st_Baron_Avebury