In Doctor Who episode S10E4 Knock Knock, we encounter some mysterious bugs, named Dryads by the Doctor, that may be alien, or not (we really know very little about them, sorry).

They live in the walls of the house, or, they are the walls of the house. You know, in a "Part of the crew, part of the ship" kind of way. And they eat people by drawing them into the walls and then, presumably, slowly devouring them. So far, so good.

And in the end, it turns out that they are needed by the landlord to

keep his daughter alive (OK, OK, not really his daughter. Spoilers, spoilers), because long ago, when his daughter was dying, they saved her life. And they have been keeping her alive ever since.

But, how? What did they do? That wasn't clear at all, only that they did. Not a word about how they did it.

1 Answer 1


They take matter and energy and transfer it to Eliza

It seems that the main purpose of killing all those people was to feed the lice:

THE DOCTOR: When you saw what the creatures had done, you understood, didn't you? The lice could keep your mother alive, if you protected them, tamed them, fed them.

The lice acquire matter and energy from those they feed upon:

LANDLORD: I'd be more concerned for yourself, Doctor. Your advanced age means you have less energy, less matter. But they will take what they can get.

It would seem that, having formed a symbiotic relationship with the lice, Eliza receives some of that energy or matter they consume. Thus she glows kind of golden when the Dryads consume Shireen. The lice are maintaining her, just as they initially did: by replacing her with wood.

DOCTOR: You leave your daughter alone for the night, or so you believe. The music wakes them. They set to work. And in the morning, you find her revitalised. Just slightly wooden. You realise there's a way she can survive.

Of course, by now it is simply replacement of previous wood.

  • Yes, but simply eating people to stay alive and excreting wood as a side-effect is not quite the same as changing a person into a wooden replica of themselves, is it? Maybe the question is more of a why. Why would they keep her alive instead of just eating her?
    – Mr Lister
    Commented May 9, 2017 at 13:17
  • @MrLister - It seems that they formed a link with her. Note that she was able to control them.
    – Adamant
    Commented May 9, 2017 at 15:55

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