Alzarian Spiders are a species of arachnid-like creatures from the swamp planet Alzarius. This is a world where all life forms share a deep genetic link and evolution takes place at seasonal waves, hence new Spiders eggs can spontaneously generate inside Riverfruits fallen during the period known as Mistfall (which happens every fifty years approximately). These Spiders may also spontaneously mutate into Marshmen. When examining them under the microscope, the Doctor noticed that the cells of each Alzarian species looked exactly identical.
The Alzarian Spiders are roughly rat-sized creatures and resemble a robust Earth spider, despite having only three pairs of legs rather than the usual four. Their bodies are black with a red abdomen, and they possess three pairs of large dorsal eyes and a pair of bright-yellow bioluminescent globes on their faces. Their bite can infect other creatures with a protein that alters their behavior, causing them to become aggressive and act like a Marshman.
- Doctor Who season 18 - Full Circle (1980)