Alzarian spiders had large black bodies with red abdomens and three glowing eyes. They hatched out of the riverfruit during Mistfall. The Marshmen seemed to fear the spiders, even though they were descended from them.
Alzarian spider venom had an unusual effect: It carried a special protein that travelled into the brain tissue and turned the victim into more of a Marshman; the effects were similar to hypnosis.
Romana II was infected by a spider during her visit to Alzarius. She began sabotaging the Alzarian starliner but was cured by the Fourth Doctor. The Doctor inspected the cells of the spider and made the link between them, the Marshmen, and the Alzarians. (TV: Full Circle)