When the Doctor nicknamed the strange insectile aliens infesting Bill's new house Dryads, it seemed like something he'd plucked from the air. But as we should know by now of someone who has lived for 2,000 years, there's meaning behind most everything he does.

In fact, he could hardly have chosen a better name for the creatures who had made their home in the wooden parts of the house, and had remained there for the past 80 years. 

In Greek mythology, dryads were tree spirits, who lived long lives and never left the trees they inhabited. In fact, in Ancient Greek, 'dry' means oak, so they were specifically nymphs who inhabited oak trees. And the floorboards and wood panels of the sinister Landlord's house look very much like oak to us.

Doctor Who continues on Saturday nights on BBC1