Shadow Bugs are the small, glowing, violet spheres of which the members of the Subspace Army are made. They are apparently embodiments of a primordial substance. Shadow Bugs were created by extracting a mysterious substance from Mr. Game & Watch, leading Tabuu to have him captured to become the source of the Subspace Army's soldiers.
They first appeared in a Smash Bros. DOJO!! update called "This World", which introduced the main plot of the Subspace Emissary. They were shown being dropped down from the Halberd, as well as surrounding a single Primid.
Shadow Bug Clones
Shadow Bugs are capable of copying a character's trophy and assuming his/her appearance, but are also capable of altering certain qualities such as size, as seen in the fight with the giant Diddy Kong. A copy of a character created by Shadow Bugs will have a purple tinge, be surrounded by a smoky shadow aura, and possess glowing yellow eyes. Bowser himself had a platoon of Shadow Bugs which he ordered to serve as a clone of himself, both to act as a decoy and a second-in-command.