A robotic cockroach is a metallic insect that, along with a swarm of cockroaches, were responsible for terrorizing the town of Miller's Grove, Massachusetts. Much of their terror, though, was the result of mass hysteria over a series of deaths connected to the cockroaches.
The robotic cockroaches were believed by Agent Fox Mulder to be alien robots, while others thought the government was behind their creation. Ultimately, it seems they were probes from an unknown alien civilization.
When Mulder gave one of the robotic cockroaches to a robotics expert, Dr. Alexander Ivanov, he inspected it under a microscope. A profound look crossed his face, as he turned to Mulder and said, "It's beyond my comprehension." (TXF: "War of the Coprophages")