Crumbugs are very small even for Breadbugs, looking much like a square-piece of bread. Their armor is notably weaker as well and can't withstand as much power as other Breadbugs typically can.
Unlike many breadbugs, but much like Dwarf Bulborbs, Crumbugs actively hunt their own food, though due to their small size they usually hunt down small animals close to their own size such as Sheargrubs, Pikmin, and juvenile Amphitubers.
Of course, they are constant prey to many other animals as well. However, they have been observed in a symbiotic relationship with the Emperor Bulblax.