When you get the "Spider Gun" from Isaak, shoot in secondary fire mode. It will shoot out a small egg. If an enemy gets close to it, the egg will split and shoot Araknids on your opponent. However if you shoot the egg with primary fire, the egg will enlarge and a larger Araknid will appear. You can also get a bigger Araknid by holding [Alt] + [Fire] for a few seconds, then shooting it.