The Zilgans were massive creatures, much larger than the spiders of Earth. Some were only the size of a human, while some had legs alone which were six feet long. Like true spiders, the Zilgans made sticky webs to trap their prey. Since they lived underground, they were sensitive to light.
The Zilgans were originally worshipped by the Sensorites, as they were the most ancient creatures on the planet and they lived in the dark that the Sensorites feared. They would feed their criminals to the Zilgans to appease them and to harm a Zilgan was a severe crime.
The Doctor came to the Sense Sphere, accidentally arriving in a cave of a Zilgan. Though it was attracted to the TARDIS, he was able to blind it with a light and escape back into the TARDIS. He then mixed up a syringe of prussic acid and killed the Zilgan. When he later encountered the Sensorites, they captured him and planned to kill him for his crime. Though he tried to convince them to get rid of the Zilgans, they refused to listen to him.
When the Doctor escaped the Sensorites, he accidentally attracted more Zilgans, including one which attempted to pick up the TARDIS. He was able to escape, though accidentally cutting off the leg of a Zilgan who was trying to get into the TARDIS. (PROSE: The Monsters from Earth)