The Itsy-Bitsy Spider
Has very big eyes and is a lot smarter than you think
By MARGARET WERTHEIM
Wednesday, March 1, 2006 - 3:00 pm
In the current version of King Kong, the mighty ape does battle with a
giant spider, a recasting of a legendary scene that was cut from the original
at the last moment. Unlike apes — for whom, amid the carnage, we
can still feel sympathy — spiders have always been the cinematic
equivalent of cannon fodder, alien agglomerations of too many legs, too
much hair and too few brain cells to elicit any resonance on human heartstrings.
Yet it turns out that these eight-legged arthropods are a whole lot smarter
than we have imagined — some at least are capable of almost mammalian
intelligence. In New Zealand, a group of researchers studying how spiders
see the world have been making movies specifically for arachnids, projecting
onto screens the size of postage stamps miniature CGI extravaganzas of
Frankensteinian monsters.
“It’s quite awesome, really,” says Dr. Simon Pollard,
curator of invertebrate zoology at Canterbury Museum in Christchurch.
“You show someone these 8- to 10-millimeter animals, and they have
these two great eyes looking up at you and they are capable of watching
cartoons on television. What’s going on in their brains we have
no idea — that’s what we’re trying to work out.”
Most arthropods could never make sense of a film or video image —
their eyes and brains are far too simple — but the spiders Pollard
and his colleagues study (the so-called jumping spiders) have an astonishingly
developed visual system that allows them to respond to such images. “We
have this idea that bugs are hard-wired and pretty thick, yet what we
see in an animal like this is a lot more plasticity than was thought possible,”
Pollard notes. “They are capable of some pretty complex decision
making.”
Pollard is one of the world’s leading spider biologists and has
been studying vision-based cognition in arachnids for more than 25 years,
along with fellow New Zealander Robert Jackson, professor at the School
of Biological Sciences at the University of Canterbury. In 2003, the pair
discovered a new species of jumping spider with off-the-charts visual
acuity. It specifically preys on blood-engorged mosquitoes and lives in
an area around Lake Victoria. Here, the sky often fills with clouds of
lake flies so thick and extensive they block out the sun, yet from this
seething swarm the spider can pick out a single mosquito.
All this is possible because jumping spiders have a vision system that
gives them an acuity approaching that of mammals: Their hunting strategies
are often compared to cats’, and their eyesight is on a par with
that of domestic felines. It is orders of magnitude better than any other
insect’s and just five times less acute than human vision.
In some ways these tantalizing creatures see better than we do: For a
start, they can see from the back of their heads. That’s because,
in addition to the two primary front-facing eyes, they have six additional
eyes located around their craniums. Though not particularly acute, these
secondary eyes allow them to detect motion in a 360-degree sweep. But
it’s the primary eyes that are really stunning, says Dr. Duane Harland,
a former graduate student of Pollard’s who is now a research biologist
at Canesis, a scientific R&D company in New Zealand. “Because
jumping spiders are so small, their heads are too tiny to contain a large
spherical eyeball like ours.” Instead, Harland says, “The
spider compensates by having what amounts to a telephoto lens mounted
on a long tube.” Its eyes stick out on stalks, and in addition to
the main lens at the front of the eye, there is a second lens at the far
end of the tube near the retina. It’s the same principle that’s
behind the Galilean telescope and a similar strategy to the one that has
evolved in eagles and falcons.
The difference between a jumping spider and an eagle is that the spider
is tiny — just a few millimeters across. Its eye is tiny, and its
retina is tiny, so that at any moment it can see only a very narrow field
of view. As Pollard explains, “It’s a bit like going to the
Louvre with a pair of binoculars and looking for the Mona Lisa. Fortunately,
you’d only need to see her smile to recognize her, and if you were
a predator that ate Mona Lisas, that’s all you’d need to recognize.”
It turns out a lot of predators are like this, including jumping spiders
— rather than recognize whole scenes as humans do, they focus on
specific features and base their response on these.
Bug-eyed: Jumping spiders feature a big lens plus a smaller one at the
rear — the same principle as the Galilean From Complex Worlds, Ed.
Frederick R. Prete
Bug-eyed: Jumping spiders feature a big lens plus a smaller one at the
rear — the same principle as the Galilean From Complex Worlds, Ed.
Frederick R. Prete
When scientists first began studying arachnid behavior, their modus operandi
was to set up lures — dead prey (such as flies and mosquitoes) mounted
in various positions and stuck on pins. In order to find out which parts
of the prey’s anatomy the spider’s visual system responded
to, they started cobbling together insectoid Frankensteins: say, a spider’s
legs attached to a fly’s body, or a fly’s wings attached to
a spider’s body. But in the late 1990s, UC Berkeley biologist Dave
Clark had a curious experience — he was sorting through some spider
slides in his office when he noticed that the sunlight coming through
the window was casting an image of a particular slide onto his desk, whereupon
an actual spider began stalking the image; it could apparently recognize
what it was seeing. So began the era of spider TV.
Spider researchers still use lures, but there is only so much cobbling
you can do under a microscope. (The making of the lures is so “fiddly,”
Pollard says, that he depends on a man in Kenya just for the purpose.)
These days scientists supplement actual lures with virtual ones, using
movie-industry animation software to concoct mutant horrors for their
arachnid audience. Their silver screen is about the size of a 35 mm frame,
and it’s generally placed at the top of a ramp. “Jumping spiders
like to climb things,” Pollard notes, “so if you give them
a ramp, they’ll run up it. And if they encounter what they think
is prey, they will begin stalking.”
The reason to study jumping-spider vision, Pollard adds, is not merely
to understand what the spider sees, but to try to get a handle on how
the spider’s mind works. The goal, he says, is “to know the
mind of a spider.” Harland has recently been awarded a large research
grant to develop computer models of the vision systems of a particularly
cunning spider known as Portia. Roboticists are especially interested
in the work, which may shed light on how to build better robots. “Something
with a brain this small shouldn’t be able to do these kinds of things,”
Harland says. Portia’s ability to strategize an attack, to work
out complex paths to reach a target, or even to engage in tactics of deception
go well beyond our usual view of an insect’s capabilities. “Our
normal understanding of brains doesn’t allow this kind of behavior,”
says Harland. “So how do they do it?”
Harland is now beginning to model Portia’s vision with virtual robots;
he will program these agents to see as the spider does and fine-tune the
software as he learns more. The traditional view of spiders, Pollard writes
in a forthcoming book, is that, “being so small and primitive,”
they must be automatons. In view of recent research, that conclusion has
to be re-evaluated. “It is clear to us,” he continues, “that
when we look into Portia’s dark, bulging eyes, the lights are on,
somebody’s at home, and a lot more than an eight-legged automaton
is staring back.”
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