Arachnoid is a fixed screen shoot 'em up where the player takes control of a spider that has to defend its eggs from hostile ants. The game is most similar to Centipede. The ants attack in rows, moving horizontally one line at a time until they get to the player who moves around in the lower sections of the screen. Unlike Centipede there are no obstacles when starting out, but as the player shoots his web at the ants a wall of webbing is created. These causes the ants to change their direction. In addition to ants there are gnats and flies that can carry ants and open up passages in the wall. There are also wasps that moves across the screen at times. The game goes on until the eggs hatch or they have all been taken by the ants. Once the eggs hatch the player moves on to a new and tougher level.