The acid present in an ant sting is methanoic acid.
Methanoic acid is also known as formic acid. The chemical formula is $\mathit{HCOOH}$. Methanoic acid is a weak acid that partially dissociates in an aqueous solution and occurs naturally in bee and ant stings. It is a corrosive acid and has toxic effects on the human body. It causes a burning sensation wherever the ant stings on the skin; The effect of methanoic acid can be neutralized by rubbing a mild base like baking soda solution on the stung area of the skin.