Metal spider

The Metal Spiders were used by Cassandra O'Brien as infiltration, observation and sabotage devices.

History
The Metal Spiders were small robots with four tentacle-like appendages and two saucer-shaped body parts. The top part had a red eye which could emit light. Individually, they weren't very strong or dangerous, but could be formidable in large groups. Their small size, high mobility and large numbers made them highly effective as sabotage devices. The Metal Spiders were transported by metallic orbs, which were in turn transported by the Adherents of the Repeated Meme. Cassandra used them as a means to destroy Platform One, destroying the systems which protected it from the Earth's destruction. (DW: The End of the World)

On New Earth, Cassandra used the metal spiders as a form of surveillance both inside the New New York Hospital and outside it. She used them to spy on the arrival of the Doctor and Rose Tyler to New Earth. (DW: New Earth)