The Metebelis Spiders

The Metebelis Spiders giant spiders from Metebelis Three. The original spiders traveled to Metebelis in an Earth Spaceship with the human colonists who became their slaves and occasional foodsource. They floated to the Crystal Mountains where their minds and bodies became enlarged and they teleported to Earth, where they possessed several humans including Sarah. Their quest was to regain a crystal of Metebelis Three that the Doctor had. The entire spider colony was killed with the death of the Great One. The Great One is a gigantic spider who ruled Metebelis Three, it crouched in the center of a web of crystal, she needed the Doctor's crystal to complete the web, which had enlarged her mind, and body and she commanded him to return it. The completed circuit destroyed her, and the radiation from her cave forced the Doctor to regenerate into his forth body.

The Metebelis Crystal possess strange qualities, they focus powers of concentration and clearing on you mind. The Doctor gave Jo Grant a blue crystal as a wedding present, but she had to return it due to the effects it causes in the Amazon. The Metebelis Spiders exerted an evil influence when they recovered the crystal and inserted it into the web killing the Great One. 

The Metebelis Spiders were only in The Planet of Spiders.