The spider is harmless to humans but invaluable to scientists embarked on a project called the Spider Tree of Life. The project’s goal is to sketch evolutionary relationships among the 37,500 known species of spiders via a representative sampling of 500 species.

In his Origin of Species, in 1859, English biologist Charles Darwin wrote of "the great Tree of Life" that connects all organisms past and present and "covers the surface with its everbranching and beautiful ramifications."

In a contemporary manifestation of Darwin’s vision, the National Science Foundation in Arlington, Virginia, has launched the Assembling the Tree of Life project and awarded U.S. $17 million in grants last year to 25 institutions to trace branches of flora and fauna evolution.

"Spiders are the dominant terrestrial carnivore," said Ward Wheeler, curator in the division of invertebrate zoology at the American Museum of Natural History in New York and lead investigator for the U.S. $2.7 million Spider Tree of Life project. "They are all predators—they have a huge influence on the ecosystem."

The Spider Tree of Life quest has led Petra Sierwald, assistant curator of zoology and insects at Chicago’s Field Museum and a colleague of Wheeler, to prowl downtown basements. Recently she found a family of four females, four males and three "teenage" recluse spiders in an apartment building.

"The basement is a really nice place for these spiders," Sierwald said. "The heating ducts bring warmth, leaky pipes moisture, and the rats and cockroaches provide a great food delivery service."

From Chicago to Myanmar

Last summer Sierwald traveled much farther from home—to Myanmar. As a guest of the forestry department there, she taught courses on arthropod collection to local students and then found specimens of nursery web spiders and wolf and lynx spiders in a park near Yangon.

Different spiders required different capture techniques. For well-camouflaged webs, Sierwald uses an atomizer to spray cornstarch, which "paints" the webs. She finds the spider and either flicks it into a vial with a spoon or uses a suction device.

A flashlight turns up the wolf spider, whose eyes shine turquoise in the beam. "The reflections are so striking that you expect a very big spider—but often they are just little guys the size of the nail of my pinky finger," Sierwald said.

The need to retrace the Tree of Life stems from the belief that earlier methods of taxonomic classification prior to about 1970 were flawed.

Hundreds of characteristics for each species are fed into a computer matrix that compares these with equivalent features from thousands of other species. Only recently have there been powerful enough computers to support such calculations.

Spider Silk

Museums contain archive arachnid collections but new specimens need to be collected so that the DNA can be extracted for analysis. Specialists are traveling the world to seek about 500 species of spiders, a sampling of most of the 108 spider families.

Jason Bond, an evolutionary biologist at East Carolina University in Greenville, North Carolina, specializes in trapdoor and funnel-web spiders, and tarantulas. The last two summers, Bond has hunted spiders in Australia and South Africa. With a small shovel and pick he digs up the trapdoor spider burrows, which vary in length from one inch to two feet (2.5 to 61 centimeters).

In Australia, Bond collected 32 types of trapdoor spiders, including a close relative, the funnel-web—the most lethal specimen to humans, but not fully understood.

"The selection of spiders represents diversity as well as focusing on particular species whose evolutionary history is controversial," Bond said.

The Spider Tree of Life is important to science—and to the silk industry, among others. Weight for weight, spider silk is stronger than steel and any manmade fiber like Kevlar.

"The two silk genes currently being used commercially are from two spiders that are not even particularly good at silk production," Coddington pointed out.

"If you want to produce silk using spider genes then you want to choose the spider whose life depends on the strongest silk—as it stands we don’t know how far spiders have taken silk development." The Tree of Life could help determine the best silk-spinners.

The water spider’s air bell is in some ways working like an external lung," said study co-author Michael Taborsky.

Found in ponds throughout northern and central Europe, the water spider is the only spider that spends its entire life underwater.

Since the small brown arachnids are air breathers, they have adapted the air bell system to gather oxygen from the atmosphere. (See spiders spinning their deadly silk.)

Living in a Bubble

The air bell serves multiple purposes, said Paul Selden, a professor of invertebrate paleontology at the University of Kansas who was not involved in the study.

"[The water spider] uses this air bell as a place to live away from terrestrial predators and as a safe nest in which to keep her eggs and tend the young spiderlings," Selden said.

It is also used as a safe harbor for consuming prey and breeding.

Using short hairs on their abdomens and legs, water spiders trap air bubbles from the water’s surface, which they then carry back to specially designed underwater reservoirs spun from silk, the recent study found. (Related: "Gene for Key Spider-Silk Protein Found" [August 2, 2005].)

As the spider fills the web structure with air, the structure takes on a bell shape and a silvery sheen. The silk membrane allows oxygen to diffuse in from the water and carbon dioxide to diffuse out, so the spiders do not have to replenish the air supply often.

But until this recent study, scientists did not know that the water spiders also used the air bells to breathe.

Like the comic book hero Spider-Man, who shoots webs from his wrists to swing through the city, real-life tarantulas spin silk from their feet to walk on slippery surfaces, according to a new study.

"To my knowledge, no other animals are using silk for locomotion," said Stanislav Gorb, a biologist at the Max Planck Institute for Metals Research in Stuttgart, Germany.

Gorb and colleagues found that zebra tarantulas secrete tiny bits of silk from nozzlelike structures in their feet. These tethers allow the arachnids to scale vertical surfaces.

The discovery supports a hypothesis that ancient spiders first evolved to produce silk from their feet before changing to the modern configuration of producing it in their abdomens.

"It makes sense actually," Gorb said. "We know that all the extremities of ancestor arachnids probably had this possibility to adhere during locomotion, for example, or during prey capture."

Alternatively, the foot secretions may have evolved independently in tarantulas to help the relatively large spiders move around safely, he adds.

(Related interactive feature: tarantula anatomy and life cycle.)

Gorbs team reports the discovery in tomorrow’s issue of the science journal Nature.

Silk Tethers

Fritz Vollrath is a zoologist and expert on spider silk at the University of Oxford in England, who was not involved in the research.

He says silk production was long thought to have evolved from glands found on the legs of early spiders.

Until now no one had so clearly shown evidence that the glands actually exist on the feet of a living spider.

It’s not a surprise in a way that there are actually some [modern spiders] that still, if you wish, spin with their feet," he said. "It’s nice they’ve shown it."

Like geckos, spiders rely on weak molecular attractions called van der Waals forces generated by tiny hairs on their legs to attach to vertical surfaces, Gorb says.

In addition, spiders have small claws that enhance adhesion to rough surfaces.

Tarantulas use these mechanisms but likely add the silken tethers for better traction.

Gorb and colleagues discovered the tarantulas’ silk-spinning abilities by examining glass plates scattered vertically in a tarantula terrarium.

When studied under powerful microscopes, the plates revealed silken remnants where the spiders had walked.

Gorb says that the silk is likely secreted as a fluid that quickly solidifies, so that as the spider steps, the silk comes out as a thread. These threads tether the spider to the surface.

As the spider steps, the threads break in sequence "like peeling off Scotch tape from a surface," Gorb added.

Oxford’s Vollrath said the finding is an example of where "the power of modern technologies are showing us how wonderful these creatures are, how clever in solving tricky problems."

Gorb and his colleagues have yet to discover the mechanism that allows the spider to control silk generation, though one must exist, they say.

Nor do they know if the spider always uses its sticky backup when the creature moves.

"You can imagine if it’s running over the surface, this mechanism will probably not be possible to use, because the silk needs some time to solidify," Gorb said.

Heavy Steps

Gorb and colleagues studied several other spider species and so far have found tarantulas to be the only species that use silken secretions from their feet.

If the common ancestor of spiders had spinnerets in its feet, as many scientists hypothesize, then the feature apparently carried over only in the tarantulas.

One explanation may be the relative weight of tarantulas when compared to other spiders, Gorb says.

Tarantulas weigh on average 0.18 to 0.25 ounce (5 to 7 grams). The next largest spiders are only about 0.07 ounce (2 grams).

Vollrath says that the zebra tarantulas may need the foot spinnerets to navigate their native rain forest habitat in Costa Rica, which can include large, slippery leaves.

"Protein is not cheap," he said, referring to the fact that spider silk is made of proteins.

"Even if you use very little, it still costs energy, and energy is the animals’ money … So why put it in the feet unless you really need it?"

Get a taste of what awaits you in print from this compelling excerpt. One day back home, I was watching a spider spin its astonishing construction between my desk lamp and telephone (it was a slow day), and I suddenly wanted to become a spider, at least for a little while. I picked up the phone (a cataclysm for the spider) and found a climbing instructor named Stefan Caporale, who agreed to help me build my own orb web, in the corner between two climbing walls at the YMCA in Worcester, Massachusetts. Caporale fitted me out with a climbing harness and Jumar ascenders. I’d never done any rope climbing, but with a slingful of the metal clips called carabiners over one shoulder and a rope bag in lieu of a silk gland over the other, I felt like Charlotte’s Web meets Rambo. I was, of course, going to have to cheat, starting from the moment I climbed one wall, tied my first line, and looked across 15 feet (4.5 meters) of open space to the point where I’d be anchoring the opposite end. A spider bridges this span the same way it makes a parachute, by lifting its hind end and paying a length of silk out onto the breeze. This wasn’t going to work for me. It was cheating just to look. A spider knows what’s happening around it largely by touch. It relies on as many as 3,000 vibration sensors, called slit sensilla, most of them on its legs. Eberhard had e-mailed me this thoughtful advice on my web-building: “Do it (as much as you can) with your eyes closed.” Having tied my line to a bolt hanger, I climbed back down and climbed up the other wall, where I pulled my spanning line taut. Then I shinnied back out the spanning line, trailing rope behind me. The idea was to leave this rope slack and let the middle of it drop down to become the hub of the web. A spider can do this blindfolded. Then it rappels down from the hub and stretches a spoke to the bottom of the web, keeping the whole thing under tension. Creeping out into midair, 15 feet (4.5 meters) above the concrete floor, I moved by millimeters. My muscles quivered. Then I began to oscillate, until I was flailing wildly from side to side and spinning sweat in all directions. It took me a half hour to get the first spokes in place. The average orb-web spider, working at an effortless trot, would already have completed an entire web, with perhaps 30 spokes. Many spiders rush to complete their webs in the last minutes before dawn, to minimize their daylight exposure to predators and also to have everything nice for insect rush hour.

Until now there were just four known instances of spiders evolving the ability to measure and create symmetrical webs: The fifth was discovered in Peru last month, prompting questions as to how and why some spiders develop the skill.

"It’s interesting because it doesn’t make any sense. There doesn’t seem to be any advantage to having a symmetrical web, yet it evolved independently among spiders more than once," said Jonathan Coddington, a senior scientist at the Smithsonian Institution in Washington, D.C. Coddington has studied spiders for over twenty years.

"It’s not possible that this is a just random drift in evolution and these spiders are stumbling into the ability to measure things. It must have evolved for a reason, but we don’t know what that reason is yet."

The Webs They Weave

Of the more than 37,000 species of spiders, all of them can make silk, but only about half use the silk to spin webs. The rest use silk to wrap prey or eggs; weave small, temporary shelters; or create a safety line if they are jumping, Spider-Man style.

The silk emerges from short, muscular projections called spinnerets, usually at the posterior of the abdomen. "The silk is in liquid form in their abdomen which emerges as a solid thread: Researchers are still working out how that happens," explained Linda Rayor, an assistant professor in the department of entomology at Cornell University in Ithaca, New York.

"The silk is stronger than steel for its width and of course far more flexible. It can stretch to 200 times its length."

Each species of spider has a style of weaving that is both innate and easily recognizable to an expert. "Show me any web on Earth and I can tell you what species of spider built it," Coddington said. "Just like an art expert can recognize a Michelangelo or a van Gogh as soon as they see one."

However, just as each painting is unique, each web is customized by a spider to fit the specific space where they’re building. "Spiders will alter the design of the web based on wind conditions or the surrounding vegetation," explained Robert B. Suter, professor of biology at Vassar College in Poughkeepsie, New York.

Among the best known symmetrical webs are those of orb spiders. "There are around 5,000 species that spin orb webs," Coddington said.

Most regularity in nature evolved for one of two reasons. It’s either a function of growth rate, which accounts for the spiral of a nautilus shell or a snail shell, "or it’s a packing issue," Coddington said. "If there are a set number of identically shaped objects, the most efficient way to pack them is in a regular array, like a sunflower’s seeds."

Neither of these reasons explain why there is symmetry in a spiderweb. However, Rayor suspects that for webs with radial symmetry, the answer is a matter of biodynamics. For a web to be effective, it needs to be built so that an insect doesn’t snap the web or bounce out of it.

"As the insect crashes into the web, that impact has to be absorbed by the radial threads. So perhaps the advantage in symmetry is that the force is spread more evenly throughout the web and reduces the thrust, making the web less likely to tear," Rayor said

New Spider Discovered in Peru

The species Coddington recently discovered lives in Peru, though he surmises it probably exists throughout the Amazon Basin.

The species has no common name yet, so it’s referred to by its Latin name, Ochyroceratidae. Adults are an iridescent blue or purple, less than half a centimeter (one-fifth of an inch) in length.

Coddington was in the leaf litter on the forest floor with a group of Peruvian students when he first spotted an Ochyroceratidae web.

"Leaf litter, from a bug’s point of view, is a catacomb of caverns, tunnels, and amphitheaters. There is an entire fauna that exists there," Coddington said. "It is the bottom of the food chain, where insects and the like are eating leaves, causing them to decompose and turn into soil."

For the tiny animals that live among leaf litter, something as simple as the curl of a dead leaf is like the opening of a cavern. Ochyroceratidae spiders weave webs to cover those openings and catch bugs as they are coming and going.

"When I looked closely at their webs, I realized they were made of regular arrays of fibers. But the strange thing is that the arrays are irregularly assembled," Coddington said.

He compared it to making a pile of bricks, each created identically, which are then thrown together in a random assemblage.

"The pieces of the web are regular, but the overall web is not. So these spiders have evolved this very rare trait of being able to measure something out in regular intervals," Coddington said, "but for reasons I can’t explain, they don’t exploit that ability like other spiders do by creating symmetrical webs."

The Search for New Spiders

Coddington’s next goal is to examine whether other spiders in the same family also measure out their webs in regular intervals. "They probably do, but no one’s ever looked."

The more than 37,000 identified species of spiders probably represent only one-third to one-fifth of what is actually out there. "There are far more undescribed, unknown species of spiders on Earth than what is known. We are not even over the hump," Coddington said. "That’s what makes my job

Scientists hope to soon be able to spin spider silk without the aid of spiders—achieving an age-old human quest to harness one of nature’s most remarkable materials.

Randy Lewis is a professor of molecular biology at the University of Wyoming in Laramie. His team of researchers has successfully sequenced genes related to spider-silk production—uncovering the formula that spiders use to make silk from proteins. In the process the team acquired a better understanding of how the silk’s structure is related to its amazing strength and elastic properties.

Their next task will be using what they’ve learned to spin spider silk themselves.

"Hopefully in the next month we’ll start spinning fibers," Lewis told National Geographic News.

Scientists don’t completely understand how spiders spin liquid protein into solid fibers. With their spinnerets, spiders somehow apply physical force to rearrange the proteins’ molecular structure to turn the proteins into silk.

Understanding how spiders do this could someday result in new stronger and lighter materials that could replace plastics—and ease the cost to the environment that results from conventional plastic production. But duplicating spider silk in the lab has proven difficult.

Cracking the Code

By cracking the genetic code of spider silk, scientists hope not only to be able to duplicate the material but perhaps even to improve on it.

"We’re trying to alter both the strength and elasticity of the natural silks," Lewis said. "We’ve made a number of different synthetic genes based on what we found in natural silks—but altered in ways to make them even stronger and more flexible. We’re really trying to control elasticity, so you if come to me and ask for a certain tensile strength and elasticity, I can make a gene that will produce a fiber that does that for you."

Thomas Scheibel, from the department of chemistry at the Technical University of Munich, Germany, is engaged in similar types of "protein engineering." He recently published a review of his work in the journal Microbial Cell Factories.

"We’re now not only after the uniqueness of the silk thread but the uniqueness of the single molecular building blocks within that thread," he said.

"I would start with something in the area of paper—paper that’s strong, tough, can’t be torn. For uses like banknotes silk could be a perfect material," Scheibel said.

"In the aircraft or automobile industry, think about a material that can absorb a lot of energy. If you have an accident [that causes a dent], it might be gone hours later, because the material can take up energy and reacquire its form. That’s what happens to a web when an insect flies into the web."

Myriad of Potential Uses

Over hundreds of millions of years the 37,000 known species of spiders (and others unknown) have evolved and diversified many silks for their unique purposes. Best known and studied is silk secreted by a spider’s major ampullate glands.

Orb-weaving spiders use this kind of silk like Spider-Man, as a dragline on which to make ascents and descents. The silk is also used to create spiders’ familiar "wagon wheel" webs.

Spider silk has incredible tensile strength and is often touted as being several times stronger than steel of the same thickness. What’s even more unique, however, is spider silk’s elasticity.

"When we say spider silk is tougher than things like Kevlar [a plastic used to make body armor] that’s what were talking about. Kevlar has higher tensile strength but it’s not very stretchy," said Todd Blackledge, an entomologist at the University of Akron.

These properties suggest a potential for many applications for spider silk: extremely thin sutures for eye or nerve surgery, plasters and other wound covers, artificial ligaments and tendons, textiles for parachutes, protective clothing and body armor, ropes, fishing nets, and so on.

"One that’s initially surprising is air bags," Lewis added. "Right now an air bag just sort of blasts you back into a seat. But if it were made out of this material it would actually be made to absorb energy and really reduce impact."

"Spidergoats"

Unlike silkworms, spiders tend to eat one another and cannot be effectively farmed. That’s spawned a search for alternative silk sources. The most common method is introducing silk-spider genes into other organisms so that they can produce silk proteins that might later be used to create artificial silk threads. Host organisms range from simple bacteria to goats.

Quebec-based Nexia Biotechnologies created a stir in 2000 when it bred two "spidergoats" named Webster and Pete. The goats were altered with spider genes so that they could produce silk proteins in their milk. Nexia’s artificial silk product is known as BioSteel, but the company is currently involved in a restructuring that has stalled research efforts.

Bacteria produce enough proteins for research work, but their long-term commercial production potential is unproven. Other efforts have focused on silk-producing plants such as tobacco or alfalfa and have met with some success.

But while producing spider-silk proteins is becoming more feasible, and scientists continually learn more about how to spin them into solid materials, major hurdles must be cleared before "spider products" become available.

So far, artificial fibers have lacked real spider silk’s strength, and the artificial threads have been much wider than their natural counterparts. Before the advent of a spider-silk marketplace, human web weavers must close the technology gap on their arachnid counterparts.

Spiders and insects that eat other creepy crawlies purposely seek a
balanced diet to maintain their health, according to a new study.
Scientists found that three predatory invertebrates—all of
which use different hunting methods—adjust their feeding to correct
nutritional deficiencies.

Researchers behind the study—to be published tomorrow in the journal Science —say other, much larger predators, like leopards and sharks, may also monitor what they eat to maintain a balanced diet.

While it’s known that plant eaters and omnivores often eat a wide
selection of foods to ensure the intake of various nutrients,
carnivores aren’t thought to be that fussy. Yet the study showed that
predators also "read the label" when selecting their prey.

Scientists based in England, Denmark, New Zealand, and Israel tested a
quick ground beetle, an ambushing wolf spider, and a web-building
desert spider to see if they selectively forage for fat (lipids) and
protein.

The animals were first given an unbalanced diet, skewed in
favor of either lipids or protein. Their subsequent feeding, after they
were given a choice of foods, was then monitored.

Previously fed a lipid-rich diet, ground beetles (Agonum dorsale)
subsequently ate protein-enriched food to compensate for the imbalance.
The reverse happened when they were initially fed protein-laden food.

It was a similar story for the wolf spider (Pardosa prativaga), according to co-author David Mayntz, a zoologist at Oxford University, England.

He said, "Wolf spiders don’t build webs but sit and wait for
prey to appear and then ambush them, so we didn’t think they would be
able to go out and select their diet. They have to deal with whatever
they catch. But we found what they eat from the prey they do catch will
depend on how much protein is in the prey and what [the predator's]
last meal was. If they had a lipid-rich meal the day before, then the
next day they would eat more prey containing lots of protein."

Desert Spider

The web-building desert spider (Stegodyphus lineatus) has even less control over the type of prey it eats, Mayntz says.

"It cannot do anything to attract specific animals with specific
nutrients," Mayntz said. "It has to deal with whatever ends up in the
web."

Possible mechanisms that allow insects and other invertebrates to alter
their feeding according to nutritional need have been identified by two
of Mayntz’s colleagues at Oxford University’s zoology department,
Stephen J. Simpson and David Raubenheimer.

Working with plant-eating locusts, they identified a novel
"taste-feedback" mechanism, whereby levels of nutrients in the blood
indicate a locust’s nutritional state, giving it a taste for the type
of food it needs most.

Simpson said: "This provides direct, nutrient-specific control
over food selection and consumption and allows insects to make
sophisticated nutritional decisions without requiring complex neural
integration."

Food Odors

The locusts were also shown to associate certain smells with
beneficial foods. Simpson says they were specifically attracted by
odors previously associated with foods containing nutrients the insect
was deficient in—even after only a few hours of deficiency.

For predatory bugs, Mayntz says different food will also have different nutritional values.

"Some prey can have as much fat as a sausage, while another will be more like a lean steak," he said.

Mealworms, for instance, contain high amounts of lipids.

"If protein is needed, a predator might go for something like a
mosquito, which has a huge amount of muscle compared with body lipids,"
Mayntz added. "Mosquitoes also suck up blood, which is full of
protein."

Some larger predators have also been shown to be choosy about
what they eat. For instance, experiments with carnivorous fish reveal
they are able to compose diets based on nutritional value.

Biologists at the University of Murcia in Spain found that
rainbow trout, when offered a range of foods, went for the high-protein
option, while cutting out fats and carbohydrates. And it’s possible
that much bigger carnivores, such as leopards and sharks, show similar
feeding behavior.

Mayntz says leopards might get particular nutrients by
concentrating on certain parts of a carcass. "If they can’t eat a whole
antelope at once, perhaps they will start eating the bits they need
most and then try to hide the rest," he said.

Indeed, Mayntz and his colleagues intend to move up the food
chain in their investigation of predators’ eating habits, starting with
mink and cats.

Who knows? Maybe even great white sharks consult their diet sheets when deciding what next to sink their teeth into.

Lab experiments conducted near Lake Victoria showed the spider
preferred female mosquitoes fed with human blood over all other prey,
including male mosquitoes, which don’t feed on animal blood.

Tests of the spider’s prey preferences showed it went for
blood-engorged female mosquitoes in 83 percent of cases when offered a
choice of two similar-size insects.

When it came to making a choice based on smell alone, with the
two meal options hidden from view, around 90 percent of jumping spiders
selected the blood-filled mosquito.

Although no other predators are known to choose prey based on the
prey’s last meal, other spiders may also select their victims this way,
says study co-author Ximena Nelson, who conducted the research while at
the University of Canterbury in New Zealand.

The spider is the also first known predator that deliberately feeds on vertebrate blood by eating mosquitoes.

The finding raises the possibility that other spiders also have a taste for human and mammal blood, the researchers add.

The blood-hungry spider, Evarcha culicivora, is found
only around Lake Victoria in Kenya and Uganda. A species of jumping
spider, or salticid, it usually hunts insects on tree trunks and
buildings. It stalks its prey rather than trapping it in a web.

The study team says the jumping spider uses both its acute eyesight and its sense of smell to seek out the mosquito Anopheles gambiae, a notorious blood-sucker that is the main carrier of malaria in Africa.

The team’s findings are published this week in the online edition of Proceedings of the National Academy of Sciences. The research was partly funded by the National Geographic Society.

"Salticids are predators that actively search for prey and mates and
typically do not build webs," she said. "They have evolved eyes that
support high-acuity vision suited to their active lifestyle."

Spiders don’t have the skin-piercing mouth parts needed to feed
directly on human blood, but the mosquito-munching jumping spider
appears to have got around this. The strategy has other advantages as
well, Nelson points out.

"Blood-feeding is a dangerous activity," she said. "Animals
that are bitten have a swatting response, and often the insect is
killed."

By eating mosquitoes, the spider avoids the risk of being squashed by an unwilling blood donor.

The study team suspects a blood meal is also biologically important to E. culicivora.

They say spiders expend a lot of energy breaking solid food down into liquid by injecting their prey with digestive enzymes.

"Perhaps blood is a ready-made, nutrient-rich liquid meal," Nelson said.

Picky Eaters

Another recent study suggests spiders are surprisingly picky eaters, purposely seeking a balanced diet to maintain their health.

The study, published in January, found that spiders are
selective about the nutrients they take from their prey depending on
their need for proteins or fats.

Even the web-building desert spider (Stegodyphus lineatus),
which has little control over what it catches, was able to balance its
diet, according to David Mayntz, a biology professor at Aarhus
University in Denmark.

He says the spider likely does this by altering the cocktail of
digestive enzymes it pumps into its prey, allowing it to extract extra
protein if the last prey it ate was low in protein.

Mayntz describes the African blood-eating spider find as "a very fascinating story indeed."

While he says blood-fed mosquitoes might be easier to catch than other
mosquitoes because they are heavier, their higher nutrient value may
well make them more appetizing to spiders.

Mayntz adds that the East African spider must have evolved a special ability to handle large amounts of vertebrate blood.

"[The spiders] would need some special enzyme to deal with this more
complex protein," he said. "It’s likely they would benefit from this
protein, which is more or less dissolved already and easy to extract."

The hairy arachnid’s venom was found to include chemicals that target
the same pain pathways as chili peppers, causing maximum distress to
bite victims.

The study suggests that animals can defend themselves by
activating the sensory nerves of their enemies, just like certain
plants do.

While spider toxins that lead to shock, paralysis, and death
are well studied, far less is known about how spider venoms cause pain.
(Related video: "’World’s Largest Spider’ Stalks South American Jungles".)

In the new study, a team led by neuropharmacologist David Julius from
the University California, San Francisco, identified three pain-causing
molecules in tarantula venom.

These neurotoxins were found to activate the same receptor on
sensory nerves that produces the burning sensation animals get from
capsaicin, the "hot" ingredient in chilies.

The findings, which will appear tomorrow in the journal Nature, are based on the venom of the Trinidad chevron tarantula, a large, long-legged species from the Caribbean (map of Trinidad and Tobago).

Painful Warning

Previous research has revealed that chili plants employ capsaicin to cause pain in rodents that might otherwise eat them.

Julius and colleagues found that normal lab mice exposed to the
newly discovered spider toxin molecules acted as if in pain, and their
paws became inflamed.

However, mice genetically engineered to lack the capsaicin
receptor showed no obvious signs of discomfort and only minimal
swelling.

"This would parallel mechanisms adopted by numerous plant species to
deter predatory mammals through the production of chemical irritants,"
the team wrote.

In peppers, the defense mechanism appears to be targeted specifically
at predators that don’t benefit the plant. Bird species that are
effective at dispersing the seeds inside peppers seem to be immune to
the effects of capsaicin.

Toxic Mix

Inflicting pain is only one function of spider venom, however, scientists point out.

Venom injected via a spider’s fangs acts in various other ways,
such as to kill or immobilize prey and to begin the process of
digesting its meal.

Wayne Hodgson, head of the Monash Venom Research Laboratory at Monash
University in Australia, says animal venoms are made up of a cocktail
of toxins with different actions, which when combined are often more
effective than single compounds.

"Biological activity is often complex, and individual toxins
may have a subtle role," he said. "However, there is certainly a
synergistic effect when these toxins act in concert."

Hodgson says a number of these toxins appear to be designed to cause or intensify feelings of pain.

"I am not surprised that this tarantula venom may also have that
capability," he said. "Causing pain in predators is certainly one way
to avoid being eaten."

The study of such spider venoms is extremely important in the development of new therapeutic drugs, Hodgson adds.

"These animals have evolved their venoms over many millions of
years, and they contain highly potent and selective toxins which can
target physiological processes in humans," he said. (Related: "Toxic Snail Venoms Yielding New Painkillers, Drugs" [June 14, 2005].)

For instance, scientists have discovered that the venom of the Chilean
rose tarantula contains toxin molecules that have a similar action as
anesthetics.

Researchers at the University at Buffalo in New York found that
the compounds block the mechanical activity of some cells, a discovery
which may lead to new treatments for conditions ranging from muscular
dystrophy to irregular heartbeat.

And a recent French study described two new toxins from the Trinidad
chevron tarantula—the same species used in the chili study—that are
active against plasmodia, the parasite that causes malaria. The
research raises hopes of new treatments for the disease.

The growing interest in spiders from the medical research
community has even spawned dedicated suppliers such as Spider Pharm in
Arizona, which sells a wide variety of spider venoms. (Related story: "Spider-Venom Profits to Be Funneled Into Conservation" [August 12, 2004].)

"Remembering that these animals represent little chemistry labs,"
Hodgson said, "we would be stupid not to explore their potential."