Worm Clusters can be used as ammunition by Exotic, annelid-based weaponry, such as the Viral Proliferator and the Annelid Launcher. The worms can be collected by using empty Small and Large Beakers on piles of worms. Small Beakers can collect 4 units of worms, while Large Beakers collect 8 units of worms. Remember that empty Beakers cannot be stacked in inventory, whereas filled Beakers can be, so make sure you are aware of the nearby worm piles, or stash your Beakers in a safe place. This is by far the rarest ammunition in the game, so it's recommended to save it for the bigger fights (especially when using Annelid Launcher, which uses a lot of them). Being a PSI user really helps, as Molecular Duplication can multiply all ammo units, including Worm Clusters.