Defending The Doctor in Arachnids in the UK

Up front I will admit that this is not a great episode. It's pretty slow, the main villain is boring, and the actually lesson being taught here is so blatantly obvious a six-year-old could teach you it. However, the primary criticism I hear regarding this episode is not about how Jack Roberts feels like a generic cardboard cut-out and is only vaguely interesting as an Alec Baldwin lookalike, or how everyone who dies has zero impact given to their deaths. Instead it is always about how The Doctor kills the spiders at the end. Well, buckaroos, I'm here to tell you she was RIGHT!

Setting aside my own feelings for spiders (yuck), let's get the obvious out of the way. Jack Robert's plan to shoot the spiders literally wouldn't work. Spiders don't have a central nervous system. Or rather centralized. See, us fancy vertebrates operate our bodies a bit like a computer. That is to say, we have many different systems in place with specialized duties but they are all managed by a central controller (the brain). In all vertebrate species, if you injure the brain, you effectively damage the whole system.

Yeah... Spiders don't have brains. At least not a core centralized one. Rather, their whole body acts as a brain. Hence why shooting them in the head was a really stupid idea, as all that did was just cause the spider more pain.

This is true of most all invertebrate species. This is why things like worms and insects can be cut and half and just squirm around for hours.

The other major difference spiders have to humans is actually mentioned in the episode. Since spiders and other invertebrates don't have a centralized brain, a complex respiratory system isn't really needed either. Us vertebrates need lungs to oxygenate blood that is then sent by the circulatory system to our brain. Spiders don't really need any of that. Instead opting to just take in oxygen through pores in their exoskeleton. Effectively breathing through their skin.

The amount of oxygen available through this system is directly proportional to how big spiders and other insects/insectoids can grow. This is why millions of years ago during the oxygen bloom in our atmosphere, we had tons of massive insects.

The Doctor made it pretty clear that these spiders were not sustainable under Earth's current atmospheric conditions. Their growth was not spurred by natural means. They were mutated to this size. They have grown beyond sustainability. And they were already slowly suffocating when we first met them. Not only were they unable to breathe properly but they were also starving. None of these spiders' natural prey are large enough or plenty enough to sustain them. Hence why they started preying on humans. They didn't want to, but they had to.

Ultimately The Doctor made the right choice killing them and the right choice in how. Flamethrowers and bullets would be an ineffective and cruel/painful way of killing them. Letting them suffocate to death was the most effective and least brutal way of getting rid of them.

But, I might here you say "Why doesn't The Doctor save them? Take them somewhere else?" To which I say HOW? WHERE?

There is no place on earth that would sustain them and humans would rightly try to immediately kill them if they caught on they existed. Leaving the option of The Doctor trying to find a new home for them off Earth. Ignoring the issue that she would have to invite these things on the Tardis (At which point I am never traveling in that thing again, no thank you!) there is still a massive problem with that plan.

Even if The Doctor somehow found a planet with the perfect atmospheric makeup and a plentiful food supply for these spiders... she is still dooming an entire ecosystem by introducing a massive invasive species. She would be introducing an APEX PREDATOR to an ecosystem that is entirely foreign to them. Do you have any idea what kind of damage that would do? That would almost certainly be dooming several innocent species to extinction. In order to save ONE brood of spiders.

Best case scenario is The Doctor finds some zoological society that is willing to care for the spiders in controlled captivity. Except spiders don't really do captive life well. Thanks to territorial hunting and rampant cannibalism, spiders basically have to be kept in isolation. This is why milking spider silk is so difficult. Once you start putting a bunch of spiders in one place they just start hunting and eating each other.

So, that is where The Doctor was at with these spiders. She was in an unfortunate situation where there just wasn't a sustainable way to ever save them. Jack Roberts doomed them from the beginning. They are too big and bulky to survive comfortably on Earth and way too big and powerful to be let out in any other ecosystem where they could thrive. There was no winning situation for them. As hard as it was, The Doctor literally had no choice. So she put them out of their pain.