From the Super Mario Wiki
Coalbits are small insects made out of coal that appear in the Mario & Luigi: Bowser's Inside Story. Their name is patterned after the periodic element Cobalt (Co). They assist Choombas in battle.
Coalbits do not actually attack when fighting Bowser. Rather, they jump into the smokestack of the Choomba they are traveling with to allow it to attack multiple times in a row if Bowser doesn't inhale them quickly; they may also flee from battle. If Bowser inhales the Coalbits with his vacuum attack, Mario and Luigi must fight them.
To attack, the Coalbits line up in front of one of the Bros. and march towards him with their pincers in the air, forcing the Bro to jump over all of them at once. Coalbits are usually found in groups of three in tandem with Choomba, but sometimes can be found in groups of five. Strangely enough, though coal is flammable, Coalbits take no damage from the Fire Flower special attack or the Flame attack.
Names in other languages
||Pun of Mini and Termita (termite).
||Pun on Carbón, meaning coal and Hormiga, meaning ant.
||Pun of "Charbon" (Coal) and "Mandibule" (Mandible)
||From Kohle (coal) and Ameise (ant), also a pun on Kohlmeise (Great Tit (Parus major))
||From "formica" (ant) and "carbone" (coal)