Here's an amazing video of ants covering someone's hand, spraying acid up towards the other hand that is hovering over them. The amount of acid shooting up is incredible, coming from such tiny creatures. They are probably defending themselves from what they perceive as a threat.
According to a 2014 article in Daily Mail, "The formic acid is not harmful to humans and has the same odour as vinegar. However, it is enough to scare off larger predators such as woodpeckers and jay birds, who could wreak havoc on an ant nest."
