Edge of Destiny brings Davie and Spenser back to Santuario, along with The Doctor. And I brought in a storyline I had originally planned to do in the first Santuario story, which is the miniature alien creature invading. I originally planned that a nest of the creatures would be hibernating on Santuario. In the end, just the discovery of the planet was enough of a story and I left it.

The idea actually came from some information I found on the internet about a proposed redesigned Dalek to be used in the TV movie that Steven Spielberg was originally interested in making. This idea went down the plughole, of course and the TV Movie that was made didn’t feature any kind of Daleks. Most Doctor Who fans in any case tend to think that an American redesign of the Daleks would be a bad idea, and when the series returned Daleks looked just like they always did.

But the idea of Dalek spiders stuck with me. The ones in the proposed film were full size, but my thought was of tarantula sized Daleks with eight legs. There are of course, a couple of predecessors. There are the metal spiders that Cassandra had on Platform One, and outside of Doctor Who, there are the Replicators of Stargate SG1. Which is one reason why I switched from spiders to crabs. It was a subtle difference.

I also had them on a drifting spaceship, not on Santuario itself, and sent just the three main characters, The Doctor, Davie and Spenser, to tackle them. I wanted to get across the fact that the three of them were the main resistance force against the Dominators, and this was something of a codicil to the battle the three of them were veterans of.

It is also, The Doctor’s last battle, at least officially. For two years writing New Lords of Time stories, the Doctor has been saying that he is retired, but he never quite stops being The Doctor. Principally, this was because people didn’t want me to retire the Ninth Doctor. But, in fact, it is time. We’re on the verge of having an Eleventh Doctor, and the Ninth is becoming history. He needs to take a lesser role in these stories, which will be mostly about the new generation of Time Lords from here on. There will be stories about The Doctor and Rose from time to time, but Davie and Spenser, Davie and Brenda, or Davie and both of his sweethearts will be the regular fare, with stories involving Chris, Tristie and Sukie and Vicki as the youngest adventurers will be more frequent. This story signified the final, final handing over of the reins to Davie.

http://tardis.wikia.com/wiki/Spider_Dalek