Shoot down the spiders and complete all sixteen levels. Collect your ammo from the supplies board on the left side of the screen. The fly in the top left side of the screen gives you a colour to place on the grid below. Left click a block to change its colour to the one supplied. If 4 or more blocks of the same colour are connected they will disappear. Amongst the blocks are ammo blocks, which will become available if they reach the bottom of the grid. Remove the coloured blocks fast enough from underneath