The Can't-See-Ums (also spelled Can't-See-Ems) are recurring characters in The Backyardigans.
The Can't-See-Ums are insects so small that they cannot be seen clearly, unless they are grouped together. They are based on sandflies, which are colloquially known as "no-see-ums" or biting midges. Can't-See-Ums are only repelled by Bug-Away Spray and can be scattered by strong winds. Yucky Man is able to summon these insects and has used them in an attempt to stop superheroes from saving the world.
The Can't-See-Ums are light blue. Each insect has a two wings that buzz rapidly. When Yucky Man summons a swarm of Can't-See-Ums in "Race to the Tower of Power", he turns them green.
The Can't-See-Ums have appeared in three episodes of The Backyardigans: "The Heart of the Jungle" and "Race to the Tower of Power" from season one, and "The Swamp Creature" from season two. They can also be seen in The Essential Guide and Movie Theater Storybook.