Invertebrates
Portia Spider
Portia fimbriata · also called portia spider, fringed jumping spider
Portia is a small, scruffy-looking jumping spider that has become famous among scientists as one of the cleverest invertebrates known. It hunts other spiders — a dangerous line of work — and it does so by planning routes, laying ambushes from above, and drumming out fake signals on its victims’ webs, tuning the performance by trial and error until it works.
Overview
Portia jumping spiders live in tropical forests across Africa, Asia, and northern Australia. The body is just 5–10 mm long, mottled brown and tufted, with a ragged outline and a slow, jerky walk that make a resting Portia look like a fragment of dead leaf caught on a web.
Planning a route
Shown prey it cannot reach directly, Portia will survey the surroundings first, then set off on a long detour — sometimes taking an hour and losing sight of the target entirely — along a path that ends above the prey, where the spider drops on a silk line. In the lab it reliably picks the correct detour before starting, a task many vertebrates fail.
Playing the web
On another spider’s web, Portia plucks and taps the silk to mimic a trapped insect, a courting mate, or wind. Because different spiders react to different signals, it runs through several plucking patterns, keeps whatever draws the owner closer, and abandons what does not — experimenting on its victim in real time. It makes its final move under the cover of a wind gust that shakes the whole web.
A small brain, working slowly
Portia’s brain is smaller than a pinhead, with only a few hundred thousand neurons, yet it does genuine flexible problem-solving. It tends to work slowly and deliberately, which may be the price its limited nervous system pays for that flexibility.
Status
Portia has not been formally assessed, but the genus is widespread in tropical forest and is not considered rare where habitat remains.
Portia Spider: common questions
Is Portia really intelligent, or is it just instinct?
Portia's behaviour goes well beyond fixed instinct, which is why it is one of the most studied invertebrates in animal cognition. In laboratory tests it will choose the correct one of several detour routes to reach prey before setting off, even when the route requires it to first move away from the target and out of sight of it, a task many vertebrates fail. It solves novel confinement problems, adjusts its web-plucking by trial and error against spiders it has never met, and can retain a plan over the course of an hour-long detour. This does not mean it thinks like a mammal, but it does mean the spider is doing genuine flexible problem-solving with a brain of only a few hundred thousand neurons.
How does Portia catch web-building spiders without being caught itself?
It turns the web into a hunting ground rather than a trap. Portia walks onto the outer strands of another spider's web and uses its legs and mouthparts to twang the silk in patterns that imitate something the resident spider wants to investigate, most often a small struggling insect. Different spiders respond to different signals, so Portia probes with several vibration types and settles on whichever draws the owner closer. It advances only in stages, often freezing for long periods, and it prefers to make its final approach when wind is already shaking the web and masking its own movements, so that by the time the prey spider notices anything, Portia is close enough to strike.
What does Portia look like and why?
Portia is a smallish jumping spider with a lumpy, tufted body in dull browns and greys and an oddly ragged silhouette. Sitting still on a web or among leaf litter, it looks convincingly like a piece of plant debris or a bundle of detritus that has blown onto the silk, which lets it approach other spiders without triggering an attack. Its movement adds to the disguise — instead of the quick, darting runs typical of jumping spiders, Portia often moves in a slow, halting, mechanical way that does not read as a living animal to the spiders it hunts.
Does Portia have good eyesight like other jumping spiders?
Yes. Like all jumping spiders it has a set of eyes including two large forward-facing principal eyes that give sharp, detailed vision at close to medium range, unusually acute for an animal its size. Portia uses this eyesight to identify prey spiders from a distance, to judge the layout of gaps and supports for a detour, and to watch the target's orientation so it can freeze whenever it is being looked at. The combination of good vision for gathering information and slow, deliberate movement for acting on it is central to how the spider hunts.