I have a strange question for you. I am an American living in Austria and my Austrian friend says when an ant is crawling on you and you feel an itch afterwards that the ant has peed on you. I thought ants were biters. Do you know which is right?
This is actually a really interesting question. Ants don't technically pee: they excrete their metabolic wastes through their digestive tract, rather than through a specialized urethra. However, there are many other fascinating compounds that also come out of the back end of an ant. Many of these vary by species, and most are used in communication and/or defense. Many ants have a stinger, which they use to puncture the skin or exoskeleton of their enemies and pump poison into them (after all, ants are closely related to bees and wasps). The most painful experience you could have with an ant is getting stung by one. Some can do a little bit of damage by biting (especially very large ants like Camponotus gigas) but most ants combine biting with stinging or some other chemical defense when they are hunting or defending themselves.
Many ants do not have stings, and rely entirely on chemicals that they can secrete or spray instead. One of the most common and obvious of these chemicals is called formic acid. Formic acid is sprayed out of a specialized nozzle at the back end of ants in the subfamily Formicinae. This group of ants contains the most abundant and largest ants in much of Europe (as well as North America and northern Asia). It was the probably the conspicuous, acrid smell of the formic acid from these ants that inspired the Middle English word "pissemyre." The second part, "-myr," ultimately comes from Greek for "ant" (someone who studies ants is called a "myrmecologist"). I don't need to explain what the prefix "piss-" means to any Americans, I'm sure. I still think formic acid smells more like vinegar, but perhaps the Middle English wanted their name to be funnier.
As to whether or not formic acid, or any of the other defensive secretions other ants might use would make your skin itchy: I suspect not. I know it doesn't make my skin itchy, although it does make me cough if I accidentally inhale it. In fact, a German colleague mentioned that many people in his native Bavaria think that formic acid is good for the skin, and go to somewhat ridiculous lengths to encourage ants to spray them. It does sting slightly when it is sprayed in an open wound, of course, so ants that are really trying to inflict harm will bite you and then spray acid in the wound. Also, I suppose it is theoretically possible that someone might be allergic to something that ants secrete.
However, in practice, I suspect that the majority of people who feel itchy after an ant walks over them are just the victims of their own imagination.
I hope this helps!
Jesse Czekanski-Moir & the AntAsk Team