Whip scorpions (vinegaroons) such as the North American species Mastigoproctus giganteus - an unusual arachnid of the pet trade – spray acids at would-be attackers using a pair of pygidial glands. The recipe of this acid cocktail includes a variety of different compounds, but to date, the genetic, biochemical, and cellular bases for acid production remain a mystery. Here, we propose to investigate the genes and pathways in the pygidial glands that lead to this unique form of defense.