Nanocon

Nanocons are Decepticon drones from the IDW portion of the Generation 1 continuity family.

Nanocons are very small. Created by a Decepticon with an eye for the minuscule, they are designed to invade a hapless Autobot's systems and kill him from the inside.

They can apparently combine.

Fiction

IDW Generation 1 comics

Ultra Magnus was infected by a swarm of Nanocons while arresting a Decepticon who'd been peddling miniaturization technology. All of the tiny Decepticons were killed shortly afterwards.

That is, until they were revived and started wreaking havoc inside him again. A downsized Rodimus, Skids, Whirl, and First Aid entered Ultra Magnus' body in order to combat the swarm, but ultimately the rest of the infestation was wiped out when he did the unthinkable.Primus: You, Me, and Other Revelations