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THERE is an alien intelligence living among us. These creatures possess an extraordinary kind of consciousness, including minds that extend beyond their bodies. Yet, thanks to our ignorance and arrogance, our immediate impulse is to kill them.

This is no fantasy. These alien minds really are lurking in the shadows of our houses and gardens: spiders. We have long assumed that, like many invertebrates, they are little more than automata, lacking an inner life. But we are now discovering that some arachnids possess hidden cognitive abilities rivalling those of mammals and birds, including foresight and planning, complex learning and even the capacity to be surprised. Stranger still, the delicate silk threads they spin out behind them, so easily swept up by a feather duster, help them to sense and remember their world. Indeed, spiders’ silk is so important to their cognitive abilities that some scientists believe it should be considered part of their mind.

Now that we are starting to appreciate spiders’ intellectual capabilities, we must surely change how we see one of the most ubiquitous, important and vilified groups of animals that has ever evolved. What’s more, these incredible creatures could also challenge our understanding of our own intelligence and minds.

Spiders have deep evolutionary roots. The earliest fossil evidence of silk-producing arachnids dates from almost 400 million years ago, shortly after the first definitive evidence of insects. “Insects are the most successful lineage on Earth, but spiders pretty much follow them,” says evolutionary biologist Miquel Arnedo at the University of Barcelona, Spain. Today, there are more than 48,000 known species, with every square metre of land . That may terrify arachnophobes, but without them, agriculture would be impossible. “You couldn’t have any crops – insects would eat them all,” says Arnedo.

Spiders are defined by their eight legs and , which extrude silk. This is a protein made up of chains of amino acids, primarily glycine and alanine, that spiders extract and comb with their back legs to strengthen it. Some species can produce fibres with different chemical compositions depending on their particular need.

Spider silk – the envy of human engineers – is put to a huge range of uses, including building cocoons, windsurfing and as a rope for climbing and abseiling. Many spiders also use it for a sort of “flight” known as ballooning, in which a few threads, lifted by electrostatic forces in the atmosphere, carry them far and wide on nothing more than a light breeze. These species detect these forces with hairs on their legs, allowing them to decide when to put up their “sails”. “They can feel the change in the air,” says Erica Morley at the University of Bristol, UK, who made this discovery.

The sheer versatility of their silk helps explain the enormous evolutionary success of spiders, says Arnedo. But their intelligence will have been critical, too. The best evidence of spider smarts is found in the everyday activity of web building. Around a third of all species make orb webs, those beautifully geometrical deathtraps hanging in our houses and gardens. They hold more secrets than Charlotte’s Web, because each is a record of the decisions taken during its construction.

“Webs are like instruments that spiders carefully ‘tune’ according to their experiences”

Consider the basic question of where to build the web. Every species of orb web spider has a preferred size and shape of web to suit its body type and prey, but to fit into restricted spaces or around obstacles. This suggests that before building begins, some kind of planning takes place. That is particularly impressive when you consider that most spiders are almost completely blind. How do they do it?

One possibility, according to Thomas Hesselberg at the University of Oxford, is that they use their silk as a kind of plumb line to scout out a potential web site. When exploring, some spiders first cast a horizontal line between two surfaces and then traverse it, descending and ascending at various points along the way. This could allow them to estimate the dimensions of the space. “Maybe they’re somehow able to tell how much silk there is and how much time they spent exploring. That could potentially give an indication of how many times they passed through an area and how big it is,” says Hesselberg.

The ability to form a mental representation of space has been documented in many mammals and birds, but is almost completely unheard of among invertebrates. Until now, the best evidence was in bees, which are thought to use mental maps to help them navigate – but .

Hesselberg’s preliminary experiments suggest that the cognitive maps of orb web spiders aren’t very detailed: they seem to include big obstacles but not little ones, for instance. Yet the ability to generate even a of their environment would that appear to demand a certain level of foresight and planning. For instance, orb web spiders alter the size and structure of their webs according to the remaining silk reserves in their glands, ensuring that they don’t run out midway. They are also sensitive to the weather: in low temperatures, they make simpler structures with bigger gaps between the spirals to avoid spending too much time exposed to the cold.

Cognitive sophistication is evident in the ways that spiders maintain and adapt a web after construction, too. They in the areas most likely to trap unsuspecting insects, based on their previous catches. Increased tension boosts the transmission of vibrations from a struggling insect, allowing the waiting spider to respond more quickly. They also . They can even learn from near misses: if their prey hits the web but then escapes, spiders will to ensure this doesn’t happen in future. You could almost think of webs as instruments spiders carefully “tune” according to their experiences. “There is no doubt that they are much more flexible than we once believed,” says Hesselberg.

Spiders are a varied group, of course, but species with very different lifestyles are proving equally smart. Take jumping spiders belonging to the genus Portia. Their preferred prey is other spiders, and these can be up to twice their own size. They catch them by stealthily roaming their environment, and have evolved astonishing eyesight to do this. “They hunt like tiny cats,” says Fiona Cross at the University of Canterbury, New Zealand. This often involves premeditation and careful route planning, as her research reveals.

Mental maps

Working with her colleague Robert Jackson, Cross in a water tank – like cats, they hate getting wet. From this viewpoint, they could see two floating pathways, one of which led to prey on another tower. Although the prey was out of sight once the spiders had descended, they invariably chose the correct route, suggesting they held a map in their working memory. In another experiment, Cross and Jackson provided two routes: both led to the prey, but one was shorter. . “For us, that suggests that Portia are making a plan of where to go,” says Cross. In the wild, the spiders use this kind of premeditation to ambush dangerous prey from behind.

Portia even seem capable of being surprised. In experiments similar to the previous one, Cross and Jackson changed the type of prey while it was out of sight of the spiders. This led them to pause – , often used by psychologists to test what babies are thinking. Portia also pause for longer if they find more or fewer prey awaiting them at the end of the path, compared with what they had seen from their tower. That suggests these .

Despite Portia‘s catlike stalking style, silk is still essential to their survival, and they often employ it when hunting. They sometimes build a web as a vantage point from which to abseil onto prey, or as a means of catching insects to use as . Portia also invade other spiders’ webs and pluck the silk to mimic a trapped insect, drawing occupants from their hiding places so they can pounce and kill them. They seem to through trial and error.

Such findings, combined with new evidence of spider sociability (see “Social webs”), indicate advanced cognition across many spider species. Yet the brains of these creatures are often minute: most orb weavers, for instance, weigh between 50 and 80 milligrams, with some less than 1 milligram, and their brains are just a fraction of that. Spiders’ ancient origins also raise some profound philosophical questions about the evolution of the brain. Previously, we have tended to look for intelligence in our closer relatives, but broadening the web to animals like spiders has proved profitable. “The success of this line of research shows that we should be careful not to restrict our research questions or study species,” says Caroline Strang at the University of Texas at Austin, who investigates insect cognition.

These findings have even caused us to reconsider the definition of the animal “mind”. Take the concept of extended cognition, an idea that has been of particular excitement to psychologists of late. to describe the way our tools become incorporated in our thinking. We turn to a notebook or smartphone, for instance, as an external memory source that can be accessed at will. In this view of cognition, our minds extend beyond our brains to all the objects that contribute to our perception, memory and reasoning. Hilton Japyassú at the Federal University of Bahia in Brazil and Kevin Laland at the University of St Andrews, UK, have made a strong case that a spider’s use of silk qualifies as a . After all, a spider’s past experiences can determine the structure of its web, which, in turn, alters what it senses in its environment, directly influencing its future decisions. As a result, part of a spider’s memory and decision-making is outsourced beyond its body.

Much as tool use has aided human success, spiders might have evolved extended cognition to help them survive and thrive in varied environments. “Spiders have extra selective pressure for evolving extended cognition because of their relatively tiny brains and their high cognitive requirements as generalist predators,” says Japyassú.

“Given spiders’ smarts, might we even consider them to be conscious?”

Given the new discoveries, might we even attribute consciousness to spiders? Most researchers see consciousness as a spectrum, ranging from a basic mental representation of the surrounding environment and the capacity to shift attention between elements, to imagination, advanced self-awareness and metacognition, the capacity to reflect on your own judgements. Most invertebrates are considered incapable of even the most basic level of conscious experience, responding to situations automatically instead. But spiders’ mental representation of space and their capacity for foresight and planning appear to reflect more complex inner experiences. “We’ve seen that Portia are able to ‘think’ about what they are doing before actually doing it,” says Cross.

This is exciting because it means that spiders could provide valuable information about which neural structures are necessary for conscious experience, as well as how these evolved. In a vertebrate, the midbrain gathers sensory information and bodily signals, then integrates these to create a mental simulation of the world and the creature’s place in it. This, in turn, directs its attention and movement according to its needs, creating an “experiential consciousness” on which to build other types of awareness, such as self-reflection. “It basically underpins everything,” says Andrew Barron at Macquarie University, Australia.

Invertebrate brains take a different form, but a structure called the , argues Barron. In spiders, a region known as the arcuate body may have the same purpose. Detailed studies of spider brains have been low on neuroscientists’ lists of priorities. However, if further evidence supports the idea that these regions create an inner representation of the world, this could help us pinpoint a date for the emergence of conscious experiences – potentially more than half a billion years ago, when the Cambrian explosion gave rise to almost all the animal groups around today.

In the meantime, a greater appreciation of spider smarts might give us all pause for thought the next time we are confronted with eight-legged squatters hiding in our hallways. Written on their silken webs, we may one day find clues to the origins of thought itself.