by Mary-Russell Roberson
Quick, how many spider species can you name?
Most people can come up with only a few, and to be fair, most spider species don't have common names. In fact, there are more than 38,000 species known to science, and that's only the beginning. "That's probably about a fifth of the total," says Jonathan Coddington, research entomologist at the Smithsonian's Museum of Natural History. For comparison, there are only about 4,000 mammal species.
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| This Antilles pink-toe tarantula (Avicularia versicolor) is just one of 38,000 spider species currently known to science. Some tarantulas can live for decades. |
What do those 38,000-plus spiders have in common? All spiders have eight legs and two main body parts—a cephalothorax and an abdomen. (Insects, on the other hand, have six legs and three body parts.) As far as scientists know, virtually all spiders producesilk and venom, and are hunters, although some hunt to a lesser extent than others.
It's the differences among spiders that are truly dazzling. Most spiders are solitary, but Metepeira labyrinthea, of Chile, lives in colonies. Most spiders eat pretty much any bug they can catch, but Paraplectanoides crassipes, of Australia, eats only cockroaches as an adult and only springtails as a juvenile. Most spiders live a year or two, but tarantulas can live as long as 20 years. Most spiders let their offspring fend for themselves, but those in the family Lycosidae carry their young around on top of their abdomens for a few days.
Lots of spiders are black or brown, but some are brightly colored. Others are highly camouflaged. Phrynarachne tuberosa, of Nepal, looks like bird droppings. The European grass spider, Tetragnatha extensa, looks like a blade of grass or a twig. And Zuniga magna, of Brazil, is the spitting image of an ant.
And then there's silk. Virtually all spiders make egg sacs with it. Many spin webs with it and wrap prey in it. But different spiders have evolved other uses for silk. Some make nets or lassos with it to capture prey, and others construct booby traps with it. Some males wrap females in it during mating, and others use it to catch a ride on the wind. The diving water spider (Argyroneta aquatica), whose genus name means "with a silvery net," constructs a bubble of silk and fills it with air, then uses it as a diving bell to breathe from as it hunts aquatic insects underwater.
Given all this variety, what's really striking is how few spiders have been thoroughly studied. "At least half of the 38,000 have probably only been seen once," Coddington says. "The number we know much about—in the sense that someone has taken a picture for an illustrated guide—is maybe 5,000. Taxonomists have described the rest but we don't know much about them."
This article summarizes some extraordinary findings from recent spider research. But there are untold numbers of amazing stories still waiting to be discovered. Indeed, as Coddington says, "That's what's cool about [studying spiders]. Nothing much is known about these things."
The Brown Recluse—Deadly
. . . Scavenger?
Most people know the brown recluse (Loxosceles reclusa)
because it is one of a very few spiders with venom that
is toxic to humans. Its bite rarely kills people, but
usually creates a painful, spreading area of decomposed
tissue. Any spider with venom this strong must be a
vicious hunter, right? Actually, no. New research by
Jamel Sandidge, a biologist at the University of Kansas
at Lawrence, shows that brown recluse spiders scavenge
rather than hunt if given a choice. It's the first time
a spider has been shown to prefer scavenging over predation.
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| There can be as many as 2,000 brown recluse spiders in one house. |
Sandidge didn't set out to study the hunting and eating habits of brown recluses—his study was focused on the relative population densities in houses of brown recluses, other spiders, and insects. As the study progressed, however, Sandidge made several unexpected observations about brown recluse behavior. In houses, he saw them eating insects that had obviously been dead for a long time. He also saw them running away from insects. He has even watched a brown recluse remain motionless while a potential prey insect walked over it. In the lab, Sandidge says, "I was feeding them yellow mealworm larvae and the problem was that the mealworms were eating the spiders." Sandidge began killing the worms before putting them in the enclosures with the spiders, and the spiders readily ate them.
Sandidge's curiosity was piqued. He set up new experiments, offering the spiders dead prey and live prey. More than 80 percent of the time, the spiders chose the dead prey. "Brown recluses are very fragile and very weak," Sandidge says. They often lose legs or even their lives in encounters with live insects, which may explain why they prefer to avoid them.
Why would a scavenger need highly potent venom? "It didn't evolve to be a scavenger. Otherwise it wouldn't have venom," Sandidge says. "Scavenging is just secondary. And it's only by chance that it's so toxic to humans. It didn't evolve to kill mammals."
In and around Lawrence, Kansas, where Sandidge works, the average house might contain about 50 brown recluse spiders, but some contain between one and two thousand. Sandidge says that house-dwelling brown recluses' willingness to eat dead insects could help them maintain large populations, because most houses contain far more dead insects than live ones. Insects are attracted to houses for a variety of reasons, but often die once inside, due to injury, pesticides, or lack of food or water. A brown recluse can get by on one insect every couple of months, so a house full of dead insects could easily support a large brown recluse population.
To observe a spider in a dimly lit area, Sandidge either illuminates the area with a red light, or dusts the spider with fluorescent powder and watches it using an ultraviolet light. When asked if it's creepy being in a basement full of brown recluse spiders, Sandidge says, "I'm pretty used to it. The only thing that gets me is sometimes when they drop from the ceiling."
While not much is known about brown recluse distribution, Sandidge says that in the southern part of their range—which includes Texas, Arkansas, and Oklahoma—they tend to live outside. In the northern part of their range—which includes Kansas, Missouri, and Iowa—they live mostly in houses. "They shouldn't be in Washington, D.C.," he says, except for individuals that have hitched rides on crates or building materials shipped from other places.
There are, in fact, at least four brown recluses in Washington, D.C., in the Invertebrate House at the Smithsonian's National Zoo. Alan Peters, the Zoo's assistant curator for invertebrates, says he feeds the spiders live crickets. "We've observed with a larger-size cricket—this is anecdotal—that the spiders do run to one side," he says. "The cricket seems to be dominating the space." The brown recluse spiders are not on exhibit, but are sometimes available for viewing if a visitor asks.
Wily Portia
Most spiders are not cannibals, but a few do specialize
in eating other spiders. One group of spider-eaters
is the genus of jumping spiders called Portia,
whose 15 species live in Africa, Asia, and Australia.
The high level of skill and trickery that Portia uses to lure other spiders into its jaws is startling. "The more you study this spider, the more impressed you get," says Stim Wilcox, an associate professor at the State University of New York at Binghamton.
Portia produces irresistible "music" to catch some species of its arachnid prey. It plucks the web of the host spider, producing a variety of frequencies and patterns. Portia tries different signals until the host spider responds by turning or moving toward it. The crafty predator then abandons the musical variety show and continues only with the signal that produced the response. Says Wilcox, "Portia clearly has cognitive ability. It has evolved to learn in circumstances where learning enables it to be a better hunter." But Portia's hunting style can be dangerous, too. One slip in its technique, and Portia can become the prey instead of the predator.
Wilcox has studied Portia for more than 15 years, ever since he was approached at a meeting by "Mr. Jumping Spider of the World," Robert Jackson of the University of Canterbury in New Zealand. Jackson had recently observed Portia in the wild ("Robert's always looking at spider webs and poking at them," says Wilcox) and wanted someone to help study Portia's web-plucking vibrations. Wilcox was a natural—he's known for his studies of ripple signals produced by water striders. The two scientists have been working together ever since. To study Portia's vibrations, Wilcox attaches a stylus (picture a record-player needle) to a web. The stylus picks up the vibrations and transmits them to a tape recorder. The vibrations are then analyzed by computer.
Perhaps even more amazing than Portia's web-plucking strategies are its detouring tactics. Sometimes, in order to surprise its prey, Portia sneaks around the back way—even if it loses sight of its goal for a couple of hours. For example, Portia may spend an hour going around a tree trunk to reach its meal, but stay on task and on track the whole time.
To test Portia's detouring abilities, Robert Jackson and graduate student Michael Tarsitano recently designed some obstacle courses in the lab. They set up a series of scaffolds, placing a prey spider as a lure somewhere at the top where Portia could see it. Time and again, Portia chose the correct scaffold to reach its prey, even if that meant first traveling away from the lure spider or out of sight of it.
"Man, that's an intelligent little hunter," says Wilcox. "Other spiders can learn and solve problems and do detouring. The thing that distinguishes Portia is that it's better, more versatile, and has a greater range of tactics. It's cleverer."
Female Wolf Spiders—They
Never Forget a Foreleg
Speaking of clever, a recent study shows that female
wolf spiders can retain memories for up to three weeks.
The study also demonstrates for the first time that
the mating preferences of female spiders can be influenced
by experiences they have before they are sexually mature.
Eileen Hebets, an assistant professor at the University of California at Berkeley, studies a type of wolf spider called Schizocosa uetzi. The males of this species have brown and black patterns on their forelegs, which they wave around during courtship displays. The patterns vary tremendously among individuals, and Hebets had long wondered why.
In an earlier study using videos of courting males, female spiders didn't have a unanimous preference for one foreleg pattern over another. Hebets wondered if perhaps different females prefer different patterns, and she designed an experiment to find out. "It was a side project just based on a curiosity I had," she said. "I knew if there was an effect, it would open up all kinds of possibilities."
In her lab, she painted the forelegs of male spiders either all black or all brown, using nail polish. She exposed immature female spiders to courting males from either the brown or black group, and then isolated the females until they shed their final juvenile exoskeletons and became sexually mature; on average, the isolation lasted for three weeks. Next, she again exposed each mature female to a courting mature male. Hebets found that the females were significantly more likely to select male mates with leg colors that matched those they had been exposed to when immature. What's more, females were more likely to eat suitors whose legs were the other color.
Who would have thought spiders could remember something for three weeks? "It's pretty exciting," Hebets says. "It opens the door to all sorts of future studies." Furthermore, this study has important implications about how social experiences of immature females influence mate selection, and by extension, how males have evolved as a result of females' preferences. If foreleg patterning is a heritable trait, then the choices that females make are driving the evolution of that patterning.
Although this is the first time social experiences have been shown to affect mate selection in spiders, Hebets suspects it will not be the last. "I think that as soon as people start looking at other species, it's going to be something they're going to find," she says.
What evolutionary advantage could there be to the females' behavior? No one knows. "That's the direction that future studies will be going," Hebets says. It may be that picking a mate with a familiar pattern is a way of upping the female's chances of mating with a male that has shown himself to be a survivor—if he's the same one she saw before, he's lasted at least since then. Or perhaps such behavior makes it more likely that the female will mate with not just any wolf spider, but one of her own species. "In the fields where I collect these spiders, mature males are everywhere, and they will try to court anything," Hebets says. She is currently preparing to study what happens if an immature female is exposed to a courting wolf spider of a different species.
In general, if a male wolf spider tries to mate with a female of a different species, the female will attack. "The females are the ones in control. They are slightly larger than the males and they are cannibalistic," Hebets says. "You generally do not get mating between the species because the females are picky. If it were up to the males, there would be heterospecific matings all the time."
When Sex is Deadly
Many people have heard that female spiders eat male
spiders after mating. While this is not true for most
spiders, it is the case for some (fishing spiders and
orb weavers, for example). However, scientists have
recently uncovered an odd twist to the old story: In
black-and-yellow garden spiders (Argiope aurantia),
males die spontaneously during mating, even if the females
do not attack them.
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| Mating can be deadly for male black-and-yellow garden spiders. |
Matthias Foellmer, a biologist at Concordia University in Montreal, and Daphne Fairbairn, a professor at the University of California at Riversid, recently studied the mating habits of black-and-yellow garden spiders. A male must insert each of his palps, a pair of short appendages right behind the jaws, into a female to successfully transfer sperm. His choice of mates will affect his ability to insert both palps and achieve fertilization. If he chooses a mature female, he risks being attacked; males that pursue this option often have to beat a hasty retreat after inserting the first palp and before trying again with the second. His other option is to find an immature female, and wait for her to molt, after which she will be sexually mature. To molt, she leaves her web and hangs from a "molting thread." The male tries to mate with her while she is hanging from the thread, because her new exoskeleton is so soft that she is unable to attack. The disadvantage of this option is that competing males usually accumulate in the web of an immature female, waiting for the moment she molts.
While observing these spiders mating in the field and in the laboratory, Foellmer and Fairbairn were startled to discover that males died spontaneously within minutes of inserting the second palp. This was true whether they mated with an older, mature female or a newly molted female incapable of attack. It was even true for the male that inserted his second palp into a dead mealworm beetle larva in the web. "The same second, he folded up and was dead," Foellmer says. He adds that males sometimes court females of other species, immature females, and even discarded skins of females.
After the male dies, his body dangles from the female
spider, his palps still inflated inside her. A newly
molted female is unable to pull the male's palp out
until she leaves her molting thread and returns to her
web, which may be 15 to 25 minutes after mating. Meanwhile,
all those other males that had been hoping to mate with
the soft-bodied female are out of luck. "If he
blocks off other males during this important period,"
Foellmer says, "his paternity probably doesn't
get compromised." Once the female is back in her
web, she's likely to attack potential suitors.
Sometimes the females eat the dead males after removing
them, other times not. "Dead males sometimes fall
out of the web, or they may be eaten by kleptoparasitic
spiders—tiny spiders that steal prey from the
larger spider's web," Foellmer says.
Foellmer believes spontaneous male death among spiders to be a relatively rare phenomenon, although he wouldn't be surprised if it exists in some other members of Argiope. Furthermore, he notes that other researchers recently observed a similar behavior in a species of cobweb spider. One thing's for sure, he says: "Further research into the mating habits of spiders will continue to uncover fascinating behaviors."
—Mary-Russell Roberson is a freelance writer living in Durham, North Carolina. She wrote about reintroduced elk on Kentucky's Cumberland Plateau in the May/June 2003 issue of ZooGoer.
ZooGoer 33(3) 2004.
Copyright 2004 Friends of the National Zoo.
All rights reserved.
