Portia may be about the size of a fat raisin, with eyes no larger than sesame seeds, yet it has a visual acuity that beats a cat or a pigeon. The human eye is better, but only about five times better. So from a safe distance a foot or two away, Portia sits scanning Scytodes, looking to see if it is carrying an egg sac in its fangs… The retinas of its principle eyes have only about a thousand receptors compared to the 200 million or so of the human eyeball. But Portia can swivel these tiny eyes across the scene in systematic fashion, patiently building up an image point by point. Having rejected a few alternatives routes, Portia makes up its mind and disappears from sight. A couple of hours later, the silent assassin is back, dropping straight down on Scytodes from a convenient rock overhang on a silk dragline—looking like something out of the movie, Mission Impossible. Once again, Portia’s guile wins the day.
…Undoubtedly many of Portia’s cognitive abilities are genetic. Laboratory tests carried out by Robert Jackson, chief of Canterbury’s spider unit, have shown that only Portia from the particular area where Scytodes is common can recognise the difference between an egg sac carrying and non-egg sac carrying specimen. And it is a visual skill they are born with. The same species of Portia trapped a few hundred miles away doesn’t show any evidence of seeing the egg sac. But as Jackson points out, this just deepens the mystery. First there is the fact that such a specific mental behaviour as looking for an egg sac could be wired into a spider’s genome. And then there is the realisation that this is a population-specific, not species-specific, trait! It is a bit of locally acquired genetic knowledge. How does any simple hardwiring story account for that?
… “The White Tail can pluck, but only in a programmed, stereotyped, way. It doesn’t bother with tactics, or experimenting, or looking to see which way the other spider is facing. It just charges in and overpowers its prey with its size. Portia is a really weedy little spider and has to spend ages planning a careful attack. But its eyesight and trial and error approach means it can tackle any sort of web spider it comes across, even ones it has never met before in the history of its species”, says Harland. While Portia’s deception skills are impressive, the real admiration is reserved for its ability to plot a path to its victim. For an instinctive animal, out of sight is supposed to be out of mind. But Portia can take several hours to get into the right spot, even if it means losing sight of its prey for long periods.
…As a maze to be worked out from a single viewing—and with no previous experience of such mazes—this would be a tall order even for a rat or monkey. Yet more often than not, Portia could identify the right path. There was nothing quick about it. Portia would sit on top of the dowel for up to an hour, twisting to and fro as it appeared to track its eyes across the various possible routes. Sometimes it couldn’t decide and would just give up. However, once it had a plan, it would clamber down and pick the correct wire, even if this meant at first heading back behind where it had been perched. And walking right past the other wire. Harland says it seems that Portia can see where it has to get to in order to start its journey and ignore distractions along the way. This impression was strengthened by the fact that on trials where Portia made a wrong choice, it often gave up on reaching the first high bend of the wire—even though the bait was not yet in sight. It was as if Portia knew where it should be in the apparatus and could tell straight away when it had made a dumb mistake.
Crazy talk, obviously. There just ain’t room in Portia’s tiny head for anything approaching a plan, an expectation, or any other kind of inner life. The human brain has some 100 billion neurons, or brain cells, and even a mouse has around 70 million. Harland says no one has done a precise count on Portia but it is reckoned to have about 600,000 neurons, putting it midway between the quarter million of a housefly and the one million of a honey bee. Yet in the lab over the past few years, Portia has kept on surprising.
…Rather controversially, Li calls this the forming of a search image. Yet even if this mental priming is reduced to some thoroughly robotic explanation, such as an enhanced sensitivity of certain prey-recognising circuits and a matching damping of others, it still says that there is a general shift in the running state of Portia’s nervous system. Portia is responding in a globally cohesive fashion and is not just a loose bundle of automatic routines.
…Harland says Portia’s eyesight is the place to start. Jumping spiders already have excellent vision and Portia’s is ten times as good, making it sharper than most mammals. However being so small, there is a trade-off in that Portia can only focus its eyes on a tiny spot. It has to build up a picture of the world by scanning almost pixel by pixel across the visual scene. Whatever Portia ends up seeing, the information is accumulated slowly, as if peering through a keyhole, over many minutes. So there might be something a little like visual experience, but nothing like a full and “all at once” experience of a visual field. Harland feels that the serial nature of this scanning vision also makes it easier to imagine how prey recognition and other such decision processes could be controlled by some quite stereotyped genetic programs. When Portia is looking for an egg sac obscuring the face of Scytodes, it wouldn’t need to be representing the scene as a visual whole. Instead it could be checking a template, ticking off critical features in a sequence of fixations. In such a case, the less the eye sees with each fixation, perhaps the better. The human brain has to cope with a flood of information. Much of the work lies in discovering what to ignore about any moment. So the laser-like focus of Portia’s eyes might do much of this filtering by default. Yet while much of Portia’s mental abilities may reduce to the way its carefully designed eyes are coupled to largely reflexive motor patterns, Harland says there is still a disconcerting plasticity in its gene-encoded knowledge of the world. If one population of Portia can recognise an egg-carrying Scytodes but specimens from another region can’t, then this seems something quite new—a level of learning somewhere in-between the brain of an individual and the genome of a species… As Harland says, Portia just doesn’t fit anyone’s theories right at the moment.