Mexican Defense: Horsefly Gambit is a chess opening with the ECO code A50, part of the Mexican Defense family. It goes 1. d4 Nf6 2. c4 Nc6 3. d5 Ne5 4. f4. White opens with 1. d4 (the queen's pawn) and Black answers 1... Nf6. After 7 moves it is Black to move. The lesson on the board explains every move of the line and lets you play it yourself; it is free.