It's officially arachnicide season in the Northern Hemisphere. Millions of spiders have appeared in our homes – and they'd better be on their guard. Why do we kill them so casually?

As I opened the front door, I mentally prepared myself for the obstacle course ahead.

First I needed to fetch something from the shed – the domain of monstrous spiders the size of baby mice, who lurk in corners with just their furry, gangling legs protruding. Then I moved some old paving slabs, carefully side-stepping the scuttling, scorpion-like forms of woodlice spiders, who had made their home beneath them. Finally, I guided the new fronds of growth on my jasmine plant up the fence, trying not to get covered in the wiry daddy longlegs who stalk its vertical plains.   

Eventually my journey ended on the patio – and here there was a shock. Lying on the paving, legs splayed out wildly, as though he had fallen from a great height – was the pallid corpse of Stripy. This talented web-artist had reigned over my garden for three years, and over that time something strange had happened: I had started to like him.

Seeing him there, dethroned from his latest two-dimensional creation between my bins, he seemed somehow less blood-curdlingly alien – and more like any other animal, with a heart that had just stopped beating.

But not all spiders meet such peaceful ends. The moment we sense the pitter-patter of their tiny feet across the living room floor, or catch a glimpse of movement in the corner of an eye as they abseil down from the ceiling, they're likely to end up squashed, poisoned, vacuumed up or simply flushed away from our homes. Why do many of us kill spiders so casually, swatting out their lives with our god-like power, almost like it's a reflex?

Of course, humans routinely kill animals when it's convenient – many activities, from farming to laboratory experiments, wouldn't be possible without this unpleasant reality. But the practice of arachnicide is often described in oddly victorious terms. 

"There was a spider in the bathroom i sprayed it and watched as the spider slowly died it takes about a min to kick in really good stuff [sic]", wrote one satisfied reviewer of a spider-killing product on Amazon. "spider corpses (loads of them) on the ground/dangling from bits of web," said another, describing it as a "happy result".

A complex life form

Spider massacres like these are even more jarring when you consider that spiders and humans are not so different. Though our evolutionary paths diverged at least 530 million years ago, we share many of the same organs and body parts – such as kneecaps – and similar brain chemicals, from dopamine to adrenaline. No one has ever studied spider emotions directly, but it's easy to imagine that they might be more relatable than you would think.   

Equally, their brains are hardly heavy on their spider shoulders (or in their legs – there sometimes isn't room for a full brain in a spider's head), but there's increasing evidence that some kinds are capable of remarkably complex intellectual feats, such as planning strategic detours to trick their prey. They may also have their own unique kind of intelligence, in which they're able to use their webs to help them think.

And if they can avoid being hunted down by hysterical humans, there are spider species that can live for decades.

The oldest-known spider on the planet – a trapdoor spider known as Number 16 – made it to the venerable age of 43. That whole time, she had just one home, a burrow in North Bungulla Reserve near Tammin, south-western Australia. At the time of the arachnid's death in 2018, Leanda Mason – an ecologist who had spent years studying her – told the Washington Post how she had been tragically "cut down in her prime", killed by a sting from a parasitic wasp.

If we needed any other reason to consider a spider life precious, there's the fact that – just as with all animals alive today – each individual arachnid is the product of an unbroken line of successful ancestors that stretches back to the first life on earth 3.8 billion years ago. Against staggering odds, that particular spider's parents and grandparents and great-grandparents – and so on, for millennia – managed to survive long enough to reproduce. Then a heavy-footed human steps on the spider in the bathroom because it's a minor annoyance.

So, are we all just cruel – or is there something else going on?

According to Jeffrey Lockwood, there are a number of reasons we struggle to empathise with spiders – in fact, these unlucky creatures possess a constellation of separate features that chance has combined into a package we find uniquely repulsive.

"I tell people that sort of the perfect storm of evolution and culture is spiders – well that's one, and if you were going to pick an insect it would be bedbugs," says Lockwood, professor of natural sciences and humanities at the University of Wyoming and the author of The Infested Mind: Why Humans Fear, Loathe, and Love Insects. 

An ancient enemy

Perhaps the most obvious reason we view spiders as fair game for crushing is our pathological fear of things with eight legs, which makes empathy particularly challenging. 

Human infants as young as just five months old tend to be more threatened by images of spiders than those of other organisms, suggesting that our aversion to them is partly innate, perhaps having evolved to prevent us from casually picking up ones that are venomous.

This natural wariness is then thought to be compounded by cultural factors, such as having parents who describe them as frightening as we grow up. Alarmist news articles and other depictions are likely to add an extra frisson of panic – some experts have linked the irrational fear many people have for sharks to the 1975 film Jaws, and it's possible that the villainous spider trope is also having an impact.

However, fear is not the full picture – especially because we don't have the same aversion to other arthropods that are equally dangerous.

One possible explanation is just how extra-terrestrial they seem, with unreasonable numbers of eyes – up to 12 – too many legs and toothless fangs. Their behaviour is also strikingly different to that of most mammals – building webs to trap unwary passersby, then mummifying them and sucking out their insides, or eating their mates and casually producing whole swarms of offspring.

"And so their unfamiliarity binds with a kind of commonness so that they're – I think the technical term is we find them creepy," says Lockwood.

Spiders are genetically alien, too. Though humans and spiders are distantly related, we are much less closely related to them than we are to other animals such as mammals or even reptiles.

This is potentially problematic, because the more we have in common with others – or the more closely related we are – the more compassion we have for them. One 2019 study found that participants' empathy for animals decreased in line with the amount of time since our evolutionary paths diverged.

Even scientists are heavily biased towards studying more charismatic, relatable animals. One 2010 study found that, for every research paper published about a threatened amphibian, there were 500 about an endangered large mammal.

Together, this heady mix of society-wide fear and alienation may fundamentally be inhibiting our ability to care for spiders, in much the same way that these factors marginalise minority groups of humans. For example, one experiment found that exposing people to alarming images made them less empathetic, but only towards the suffering of people outside their community. 

In fact, the combination is so effective at reducing our empathy, it's a classic way to stoke hatred towards the enemy or amplify xenophobia in wartime propaganda – and spiders are often used to achieve both simultaneously. In one iconic British World War Two poster, Hitler is shown as a gangly arachnid straddling the globe, accompanied by a visceral threat: "One by one. His legs will be broken".

A loss of control

Many of the most chilling stories about spiders have an element of surprise – such as the time a friend donned an old Halloween costume that had been stashed away in the loft for years, and someone said "wow, I love the spider detail on your neck! It's so realistic…". Much screaming ensued, because this was most certainly not part of the look, but a real spider who had silently slunk down from their hat.

Lockwood explains that this likely to be a major factor in our aversion to spiders – their ability to defy our control. "We think we're in charge of our world, but the spiders and insects keep coming," he says. "They keep crawling out from under the floorboards and whatnot. And so we have a sense – I think it's a good sense – that we don't get to call all the shots."

Spiders are simply too good at hiding and too fast. Unlike with stray pigeons or foxes, you can't simply build a wall to keep them out.  Though we now live in a world of ultra-control – where we can even turn on the heating from the other side of he world – they're still turning up unexpectedly in our beds and shoes, as they have done for millennia. Like it or not, they are impossible to eradicate from our homes.

A silent scream

There are also more practical considerations. One is that spiders don't have emotions that are easily recognisable to us, or make noises that we can relate to – so when a spider is in pain, we are oblivious.

"That's the weird space that they occupy," says Lockwood. "If they were completely unrelatable, then I think we wouldn't have such difficulty with them. But there is a sense of animal-ness about them – we recognise them at one level, but then we completely are unable to relate to them at another level," he says.

As the ecologist Stephen Kellert wrote in his book Kinship to Mastery: Biophilia In Human Evolution And Development, "...perhaps the most disturbing, these creatures [insects, spiders and other invertebrates] appear to lack a mental life". He explains that, to us, they don't seem to experience human emotions – it's as though their own minds are irrelevant to their existence. (Though of course, this is an illusion.)  

All this means that it's possible to squash a spider without receiving any feedback whatsoever – and unless your target runs away or the gruesome task is incomplete, there aren't many signals that you're doing something unpleasant. 

However, this is only true when they're small – and many people struggle to kill larger ones.

"Here we have huntsmans, which are these massive spiders like this big," says Greg Neely, demonstrating around six inches (15cm). "But here people don't kill them, they're our friends. You might try to shoo one out of the house, but you wouldn't murder it," says Neely, professor of functional genomics at the University of Sydney, where he studies pain in fruit flies, among other animals.

Lockwood can relate to this. "The same thing can be true of some very large cockroaches – my wife was pretty much normally averse to insects, but she would not, and I know others that do not like to, step on a cockroach because they make that crunch," he says, explaining that – many years ago – his wife would catch the insects under yoghurt pots for him to despatch when he got home. "It was like a little death row," he says.

"There's something about crushing another being that we don't have a big problem with until we get some sort of sensory signal, which says that this is an actual killing or this is violence," says Lockwood.

A villainous face

Apart from their menacing fangs and scampering legs, spiders face another challenge in the looks department, at least from a human perspective: they don't look like human babies.

The "babyface effect" is a hugely influential hidden bias among humans, which means that we accidentally treat people – and animals – with naturally "neotenous", or child-like features as though they are actual babies. For example, oversized eyes, large foreheads, small noses and chins, and cherubic little lips can trigger powerful feelings of empathy, compassion and affection in humans.

However, the effect can also lead us into a number of well-documented blunders. In environmental conservation, it's often observed that "cute" species receive significantly more attention and funding, while "uglier" animals under the care of humans – in zoos and laboratories, for example – may have a lower quality of life, because we find it harder to identify their suffering. 

With their tiny, beady eyes, spiders fall into this category – to our flawed ape-brains, they have a decidedly un-cute adult appearance, to which we ascribe off-putting qualities such as being less kind, trustworthy and warm. In contrast, those few spider species that have larger eyes are generally seen as less frightening.

One example is jumping spiders, which have two main doe-eyes, with their extra pairs hidden towards the backs of their heads. They’re so likeable that the researchers of one paper described them as "gateway spiders" – useful ambassadors for their less wholesome relatives. They've already starred in a number of viral videos, which set their endearingly silly mating dances to songs like the YMCA.  

A flawed hierarchy

Regardless of why we view spiders as a disposable nuisance, there are plenty of arguments for treating their lives with more respect – and psychological tricks for helping us to do so.

One is to anthropomorphise spiders as much as possible, to hijack the brain's natural compassion for other members of our species – a method which has been suggested as a way to get the public to engage more with the conservation of endangered animals (which, incidentally, many arachnids are). This can be achieved by depicting spiders with more human-like physical features, giving them relatable personalities, emotions and genders – "look, she's so cross!" – and focusing on our similarities rather than differences.

However, there's also an opposing school of thought: since empathy is so deeply flawed, we should try to avoid using it as the basis for our moral decisions altogether.

Instead, we could ascribe spider lives with value based on rational calculations – such as their function within an ecosystem. So we could try reminding ourselves that arachnids kill around 400-800 million tons (363-726 tonnes) of prey each year, and in doing so help to keep certain insect populations in check, including those that cause disease in humans.

Alternatively, we could avoid killing spiders because of their abilities and biology – arming ourselves with facts about the ingenuity of their brains, which are capable of remarkably complex decision-making, though they're tens of thousands of times smaller than those of mammals.

But there's also another way. 

"I think people assume that some forms of life are worth more than others... but they don't think about it. They don't ask themselves those questions," says Geraldine Wright, professor of entomology at the University of Oxford. She points out that our disregard for spiders is partly a Western peculiarity, since some cultures and religions – such as Buddhism – have held the view that all living things are precious for millennia.

However, some experts are hopeful that we are gradually becoming more accepting of spiders and insects – especially as people start to think more about biodiversity and environmental conservation. "The other change is the idea that each individual has an intrinsic value, and you shouldn't just kill it for the sake of it," says Donald Broom, emeritus professor of animal welfare at the University of Cambridge.

Just as I was writing this article, I noticed a new Stripy setting themselves up in my garden – perhaps one of my old friend's descendants. I think I'll leave them to it, whether I like them or not.

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