The hypodermic needle slides gently through the skin of Patient No. 799011. She's one sick spider, and this medical team is trying to save her life.

She's a Theraphosa blondi, also known as a "Goliath bird-eating spider"--though this one doesn't eat anything bigger than the occasional baby mouse. The South American tarantula is generally considered to be the biggest species of spider in the world, but even then, against the scale of her body, it's a little like a human getting an injection from the sharpened end of a garden hose.

The National Aquarium in Baltimore picked her up in April from a California tarantula breeder who saw the dime-size lesion on her side and realized there was nothing he could do for her. The veterinarians at the aquarium here have gone all out to save the spider: So far they have performed two CAT scans, consulted with experts around the world, administered antibiotics and antifungal medications, and performed any number of unpleasant medical procedures to get tissue samples.

Why all the effort to save an arthropod that most people probably think would be best treated with the bottom of a boot--or, in the case of this giant, a really big coffee-table book? "Every animal here gets equal care," says Brent Whitaker, director of animal health at the National Aquarium. Every animal from frogs weighing a few grams to whales that tip the scales at thousands of pounds.

Besides, this is no ephemeral fruit fly, pesky today and gone tomorrow. A good tarantula will be around for a quarter-century, given the right kind of care and luck. This is about saving a life, however alien to most of us; it's worth it to the staff here to try to tackle her mysterious illness, and to learn more about spider medicine from the effort.

As Whitaker withdraws the needle, pale blue fluid spurts from the injection point--hemolymph, the spider's blood. Your not-so-fearless reporter, pressed into double duty as a surgical aide, presses a cotton swab against the spider's side for a few moments until the bug juice congeals, sealing the wound.

Whitaker looks at the fluid that he's drawn into the needle and pronounces himself pleased with its clarity. During the worst of the spider's crisis, he says, the fluid was full of white flecks and looked "really nasty." He puts skin scrapings from the tip of his scalpel into test tubes half-filled with gooey-looking stuff that fungus and bacteria can grow on; the scientists hope to cultivate whatever is ailing their patient so they can target the treatment more effectively.

You can tell she's been feeling poorly by the way she's down at the heels. This is literally true: Spiders are hydraulic wonders, pumped up with hemolymph and using variations in internal pressure to move their legs. So when a spider's a little low on fluid, it's also a little low to the ground, resting heavy on the ol' footpads and nearly dragging the abdomen along the ground. We know the feeling.

Operating on a spider, however, is dicey work. Her fangs are long and sharp, and she sheds ultra-fine hairs from her rump that work their way into your skin, causing an agony of itching. More important, nobody seems to know much about arachnid surgery; there's not a lot of call for that sort of thing. The spider's own body presents major problems for the would-be surgeon, since the leathery exoskeleton is the only structure that the invertebrate has. So wielding a scalpel on this beast is "kinda like operating on a balloon that's full of fluid," explains Sandy Barnett, the herpetologist who handles the spider during all of the procedures. They had hoped the spider's body had walled off the infected area, but the CAT scans showed this was not so.

Scenes like this play out every day in the world's zoos and aquariums, where veterinary science is constantly grappling with new mysteries, says Lucy H. Spelman, associate veterinarian of Washington's National Zoo and a member of the American College of Zoological Medicine. Spelman is currently treating an elephant with an abscess in its foot, a couple of snakes with skin infections, a seal with eye problems and, of course, Hsing-Hsing, celebrity geriatric giant panda, who will take his medicine only if it's hidden in a blueberry muffin. "You have to be willing to be flexible and creative," Spelman said.

Luckily for all involved, the spider is responding well to treatment--the medications seem to be clearing up the infection underlying the lesion. What to do about that funky patch of skin, though? The staffers continue to debate their options, and hope that the tissue samples will produce evidence of a fungus that they can treat with more targeted medication. If they're lucky, the spider will eventually repair the damage herself, molting the bad stuff away as she grows.

Treating spiders does not come naturally to all of the medical staff. Ian Walker, the aquarium's veterinary intern who has spent the most time treating No. 799011, considers himself an arachnophobe. "If I weren't doing this job, I wouldn't come within 10 feet of a spider," he admits.

The operating room is actually just utilitarian backstage offices at the aquarium, its white-painted cinder-block walls a stark contrast to the colorful exhibits on the other side of the door. Barnett has the CD-size spider nestled in her leather-gloved hands, the legs pinned between her fingers. "She's very mellow," Barnett says with real affection--and a faint suggestion that she finds an afternoon spent with any given spider more fulfilling than an afternoon spent with any given reporter.

CAPTION: Though a Goliath among spiders, the Aquarium's Patient No. 799011 must be handled with care.

CAPTION: (Photo ran in an earlier edition) Staff members at the National Aquarium in Baltimore have found that treating a sick tarantula is a delicate endeavor.