Alzarian spider

Alzarian spiders were an arachnid species native to the planet Alzarius which played an integral part in to the periodic evolutionary cycle of that world.

Biology
They had large black bodies with red abdomens and three glowing eyes. They hatch out of the riverfruit during Mistfall. The Marshmen seem to fear the spiders, even though they are descended from them.

Alzarian spider venom has an unusual effect, the venom injects a special protein that travels into the brain tissue and turns the victim into more of a Marshman, the effects are similar to hypnosis. The victim acts more like a Marshman and starts to lose memories and intelligence. They also have some of the weaknesses of the Marshmen. Romana was infected by a spider during her visit to Alzarius, but was cured by the Doctor. (DW: Full Circle)