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The spiders that crawled through Jack Robertson’s hotel were genetically modified carcasses that, by toxic waste, revived and grew into enormous sizes.
Hakim Khan collected the toxic waste as “evidence” for his daughter Yasmin Khan to prove to her and the police there was something going on.
The spiders were led by a mother spider, which was larger than the others. As it was suffocating by its own size, Robertson shot it, murdering it. The spiders were eventually contained through the efforts of the Thirteenth Doctor, Ryan Sinclair, Graham O’Brien and Yaz. Arachnids In The UK