The Mutant Spiders were once normal everyday house spiders on Earth until their genetic makeup was affected by toxic waste that was discarded from a luxury hotel in Sheffield, United Kingdom. The spiders soon grew to enormous sizes compared to their average scale, from around the size of a cat to as big as a car.
Like normal spiders, they have the ability to taste and sense through their feet and communicate through pheromones. Because of their heightened senses they are able to hunt their prey through vibrations and are capable of producing webbing much stronger than that of an average spider.
After the spiders became mutated, Sheffield quickly became infested with the giant arachnids and they soon began picking off humans as a source of food.
Doctor Who Season 11 Episode 4 "Arachnids in the UK" (2018)