The Robot Spiders were used by Lady Cassandra for infiltration, observation and sabotage, They first appear in the Doctor Who episode called "The End of the World".
The robot spiders were small robots with four tentacular 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, great 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 to destroy Platform One, destroying the systems which protected it from the Earth's destruction.
On New Earth, Cassandra used the metal spiders for surveillance both inside and outside the New New York Hospital. She used them to spy on the arrival of the Tenth Doctor and Rose Tyler on New Earth. However, Cassandra's death not long after this observatory mission led to the end of their use.