Alzarian spider

The Alzarian spiders are spiders from Alzarius, first introduced in Full Circle. They are large black spiders with red abdomens and three glowing eyes. They hatch out of the melon-like plants of Alzarius during Mistfall. The Marshmen seem to fear the Alzarian spiders, even though they are descended from them.

Alzarian spider venom has an unusual effect, at least on Gallifreyans. The venom injects a special protein which turns the victim into more of a Marshman. The victim acts more like a Marshman and starts to lose memories and intelligence. They also have some of the weaknesses of the Marshmen.