Portia (spider)

Portia is a unique genus of jumping spiders known for their remarkable intelligence. They are arachnophagic, which means they specialize in hunting and eating other spiders. This behavior is unusual because they often target web-building spiders that are much larger than themselves. These prey items can be between 10% and 200% of a Portia's own size. To succeed, Portia uses complex problem-solving skills usually seen in much larger animals. They can learn from mistakes and plan their movements to achieve specific goals.

Their hunting process involves several sophisticated steps and mechanical tricks. First, many species use physical camouflage to hide from prey. They have protrusions made of hair clusters on their abdomen, legs, and cephalothorax. These growths make the spider look like leaf detritus, or dead plant debris, caught in a web. Once close, the Portia uses vibrations to manipulate its target. It can mimic the struggle of a trapped insect or the courtship signals of a male spider. By repeating these patterns, the Portia lures the victim closer.

Portia is also famous for its ability to use long, complex detours. Unlike most spiders that move in a straight line, a Portia may plan a route that takes up to one hour. They might even move out of visual contact with their prey to find a better attack angle. Sometimes, their planned path involves abseiling down a silk thread to bite the prey from behind. This suggests they possess spatial memory and the ability to plan ahead. They can even use movement cues to trick prey into revealing their location. If a prey item stays still to avoid being seen, the Portia makes undirected leaps nearby to provoke a reaction.

Scientific study of this genus has evolved significantly over time. The German arachnologist Ferdinand Karsch first established the genus in 1878. The species Portia fimbriata serves as the type species for the group. Later, a researcher named Wanless divided the genus into two distinct species groups. The schultzi group contains species like P. schultzi and P. africana. In this group, males have a fixed tibial apophysis, which is a specific part of their palps. The kenti group is distinguished by having a joint separated by a membrane on the male palp.

Modern scientists use molecular phylogeny to understand how Portia evolved. This technique compares DNA to build a tree of life. Results show that Portia belongs to a basal clade, meaning it is similar to the ancestors of all jumping spiders. Their closest living relatives include the Spartaeus, Phaeacius, and Holcolaetis genera. Some evidence of related species has even been found in Oligocene amber. Today, there are 17 described species spread across many regions. These include parts of Africa, Australia, China, India, and several Southeast Asian countries like Vietnam and the Philippines.

Despite their intelligence, Portia has a very small physical brain. The brain is smaller than the head of a pin and likely contains fewer than 100,000 neurons. For comparison, a human brain has about 86 billion neurons. To compensate, they rely on highly advanced vision. Portia has eight eyes, including two large anterior median eyes called principal eyes. These principal eyes are adapted for color vision and high spatial acuity. They can see from red to ultraviolet light. Three pairs of secondary eyes provide a field-of-view of almost 360 degrees to detect movement.

Their visual system is incredibly precise for an invertebrate. The inter-receptor angles can be as small as 2.4 minutes of arc. This makes their vision six times better than the most acute insect eye. It is also clearer in daylight than the vision of a cat. This high-quality sight allows them to recognize different types of prey and even distinguish their own silk draglines from those of other spiders. However, they are still vulnerable to large predators like birds, frogs, and mantises. These predators are often so large that the Portia fails to identify them as a threat.