According to the American Museum of Natural History, a total of 42,473 spider species, belonging to 110 different families, have so far been described worldwide. Within this spinning, many-eyed, predatory multitude, an astonishing 4,401 species—or over a tenth of the total diversity of the order—are accounted for by the members of a single family, the Linyphiidae.

Also known as sheet-weavers, because their webs consist of horizontal skeins of silk that seem to hang in the air like taut white bed linens being grasped by invisible hands, these marvelous creatures are a cosmopolitan lot. They have been observed occupying all manner of different habitats, virtually all over the world: on seashores, in deserts, hidden among the vegetation of forest floors, and even burrowed under the blankets of mountain snowfields. (Darwin himself wrote that he had seen “vast numbers” of them lashed to the rigging of the Beagle). And they're often encountered in almost fantastic profusion: nearly two million individuals, laments one guide to agricultural contaminants, can occur within a single acre of farmland.

Linyphiids are everywhere.

Yet unlike the Salticidae (the only spider family that's more speciose), linyphiids are also relatively mysterious, scientifically speaking. They're poorly studied and generally considered a taxonomic tangle that's not quite worth the enormous bother it would take to unravel. Many species, once discovered, have never become the subject of any further study beyond their original descriptions—some of which are now over 150 years old.

One reason for the family's black-sheep status in the world of zoology is that the spiders it contains are extraordinarily tiny. No linyphiid is longer than about 7 millimeters in total body length, and a great many are considerably smaller than 2mm, which brings them in under the width of some of the more exuberant fake eyelashes you can purchase. Linyphiids also tend to bear a strong resemblance to each other, so even under a microscope they can still be very difficult to identify.

These problems are particularly frustrating when it comes to the richly diverse but exceptionally minuscule spiders that belong to the subfamily known as the Erigoninae. Male erigonines, at least, have the decency to offer the vexed taxonomist some assistance in the form of weirdly modified (grooved, holey, or turreted) heads, peculiar spikes and knobs on their joints, and other obvious morphological characteristics. Males of the European species Walckenaeria acuminata even have eyes that are raised up on long, narrow stalks, suggestive of periscopes.

So much for the showy sex. What about the females? Here most arachnologists have frankly thrown up their hands in defeat. At first glance—and, perhaps, at second and third glances, too—many female erigonines look maddeningly similar: small, drab, brown, and featureless. When the 2005 tome Spiders of North America: An Identification Manual was published, the section on erigonines covered males only—the only such gap in the entire guide.

Faced with the task of distinguishing between two or more species that bear a close physical resemblance, naturalists often use rather surprising and subtle differences in anatomical features as diagnostic markers. At least one key for identifying North American bats, for instance, invites the perceptive chiropterist to determine whether the specimen in her hands is a Rafinesque's Big-eared Bat or the very similar-looking Townsend's Big-eared Bat by observing, among other things, the length of its toe hairs in relation to its claws. (A dichotomous key is a common diagnostic tool used in biology for distinguishing between different species based on a series of branching decision points.) And in order to separate the California Newt from the Rough-skinned Newt, you might find yourself peering, not into their eyes, but just below: the former typically has orange lower eyelids, the latter brown.

Spiders, as it happens (like many other arthropods), can often be identified by their genitalia. Males have highly specialized organs on their pedipalps, which are the pair of leg-like appendages on either side of a spider's face. On male spiders, pedipalps end in a shape that’s often described as looking like a boxing glove. On a male ready for mating, the “glove” inflates and specialized sclerotized parts stick out from it, like an array of tools from a Swiss army knife. One of these parts is the embolous, a tiny tube used to transfer sperm to the female. This can vary considerably in length and shape. On some male erigonines, it looks like the plunger of a syringe. On others, it can be extremely long and convoluted, spiraling like a corkscrew or, lasso-like, encircling the entire palp.

Since they are so highly detailed in structure, reproductive organs are the most crucial characters used in spider classification and identification. Unfortunately, females have been rather neglected in traditional spider taxonomy. Classification has been largely based on the male, and both descriptions and illustrations of females have often been perfunctory, when they existed at all. Nowhere is this more the case than among the erigonines. Conventional wisdom has it that even the epigyna of erigonines are too similar to tell apart. (A female spider’s genitalia is known as her epigynum, and consists of a small, hardened, shield-like plate on the ventral side of the abdomen, usually just covering the two openings to the spermathecae: her internal sperm-collecting pouches.) It has been said that rich collections of these spiders are hidden in museum collections, floating in jars labeled only “Undetermined.”

This is where Nina Sandlin enters our story.

By day, Nina is online operations manager for American Medical News, a newspaper published by the American Medical Association. But for 13 years, she's devoted one day a week to behind-the-scenes work at the Field Museum in Chicago: sorting, identifying, and organizing spiders in the museum's collections, and in the process turning an enthusiast's knowledge about arachnids into a slightly demented personal project. As Nina made her way through the museum’s nearctic backlog, sorting spiders collected from North America that had gone unidentified over the years, she accumulated a hefty jar of what she thought looked like erigonines. Using the few, treasured, tools anyone working with the Linyphiidae relied on—three enormous three-ring binders with photocopies of photocopies of the illustrations from the original literature—she made a handful of identifications that were confirmed by experts. But what she saw in the microscope was often far more detailed and striking than what the illustrations showed.

Then she got what turned out to be a life-changing email from Elizabeth, a fellow volunteer. Elizabeth was sorting through a jar of different spider family, the Hahniidae. By pointing her digital camera through the eyepiece of her microscope, she'd managed to take some beautiful photographs of her specimens, clearly showing anatomical details. Could Nina do the same for female erigonines? Erigonine epigyna, Nina thought to herself, are relatively flat—making them amenable to being characterized in a two-dimensional image.

Nina shot her first epigyna with a 5 megapixel Canon PowerShot camera, which she carefully steadied on a rolled up file folder surrounding the scope's eyepiece. The photos weren't exactly awe-inspiring, but looking at one let her recognize the same species again—and that was really all that mattered. Nina posted her "microscope field notes" to a private Picasa album, and shared it with people she'd met at arachnology meetings who were familiar with erigonines. She pestered them for loans of specimens, improved her technique, and eventually made the album public. What she now calls LinEpig is a totally free online photomicroscopy gallery of epigyna "mug shots" that collection managers can use to help them identify erigonine girls.

People who hear about what Nina does every Monday in a small back room on the third floor of the Field tend to call it "spider porn." After I watched her image Sougambus bostoniensis, Diplocentria bidentata and Diplocentria rectangulata a few weeks ago, I'd say that they're right—right, that is, assuming that the process of making adult films is also exhausting, frustrating, meticulous, and fraught with tremendous difficulty, punctuated by the occasional moment of transcendence.

Nina's moved up in the world since her file folder days, and she now creates her images with a stereoscopic Olympus research microscope and a USB digital camera attachment, hooked up to Photoshop Elements. But even after years of practice and experimentation, her workflow remains breathtakingly painstaking. Remember, we're talking about taking pictures of a single anatomical feature on creatures that themselves aren't much bigger than a speck of dirt. As I watched, the number of obstacles standing in Nina's way struck me as so numerous that it seemed amazing she manages to get any pictures at all.

Consider the following:

Erigonines are so small that Nina has to handle them using a combination of a fine-gauge pipette—with which she sucks up a bead or two of spider-containing alcohol—and an even finer insect needle—with which she spends long minutes positioning specimens. If she loses control of her charges at any point in the transfer process between the glass vials they're stored in and the little watch glasses she shoots them on, they might as well be gone forever. Nina’s misplaced at least one palp so far. "Erigonines are pretty much exactly floor-colored," she says dryly.

If you've ever tried to frame a photograph, you know that every time you think you've cleaned the glass to perfection, all the universe's dust suddenly flies towards it. That's what it's like with spider genitalia photographs, too. Flecks of old dirt from the spider's habitat cling to the hairs and joints on their bodies—looking like glowing white haloes under the microscope. To brush them off, a tool as fine as a cat whisker is required; otherwise, the entire specimen can be dislocated each time a bit of grit is dislodged. Threads from the clothing of the scientists who collected the specimens show up as huge, waving ropes that drape themselves around spider legs and heads. (Nina, muttering under her breath as she lifts yet another filament of cotton fiber: "You know, I've noticed that arachnologists really like blue sweaters.")

Nina's troubles are almost farcical in their multiplicity. Alcohol often goes cloudy when it reacts with biological material in the specimens. Each spider has eight "elbows" to get in the way of a clear view of its epigynum, and moving one out of the way can cause another to shift right back into the offending position. The particles of fine sand or electrophoresis beads Nina uses to position specimens often float above the spiders in a cloud, bury them completely, or reflect light from the two very bright bulbs that shine on the watch glass, creating glare that can ruin a good image.

And most amazing to me, if Nina works with a spider for more than a few minutes, the alcohol it's resting in can warm sufficiently from the intense light source for Brownian motion to set the dirt on her minute specimens whizzing all around the dish like balloons with the air escaping from them, while vibration from the light source jiggles the specimens themselves. (To combat this, she has to keep switching out the warmed alcohol with cooled stuff from the fridge next to her chair for each new specimen.)

Yet despite all this, and despite the fact that Nina's still unsatisfied with the quality of the photographs she gets, LinEpig is beginning to become useful.

Its photographs show, for instance, that the epigynum of Mermessus maculates (formerly Eperigone maculata) resembles, in her words, "the flaring nostrils of an angry bull." That Eridantes erigonoides has a flask-shaped epigynum with a central structure that looks like an upside-down "T." And even, amazingly, that Ceraticelus paschalis bears a striking similarity to a luxurious Dali mustache. Most importantly, grouping them together at last makes it clear that most of these epigyna do have distinct morphological differences that make them easy to tell apart.

But getting here took a lot of doggedness. When Nina first broached her proposal to Dr. Petra Sierwald, an associate curator at the museum and Nina’s supervisor, Petra was highly skeptical—with excellent reason. Linyphiids are too small, too hard, famous for being too much of a headache, and a volunteer—even an associate—would have limited time to invest. But Nina plugged away at it in her spare time, and was surprised to receive a warm reception when she presented her first 20 images at a meeting of the American Arachnological Society in 2008. The professional spider researchers cheered on her efforts, and offered their support in the most meaningful way possible—they sent her identified specimens to image. If you ask Petra about it today, she’ll tell you that Nina’s in a unique position to do this work. Most volunteers and students don’t have the "erigo-knowledge" that Nina has gradually built up, while academicians can’t spare time from the molecular work and phylogenetic tree-making that their publications require. So now the arachnid collections of America are benefited by the efforts of an amateur who is weirdly dedicated, relatively knowledgeable—and a little bit insane.

There are approximately 650 species of Erigoninae in North America. The three I watched Nina photograph brought the LinEpig count up to 193. At 200, she and Petra plan to approach some of the nation's major collections, requesting loans of rare specimens from species not recorded as having ever been collected again. It’s hard to know how many might be languishing in dusty museum jars, waiting for their photos to appear online.

*In the meantime, there’s nothing Nina would love more than for museums, universities, and individual arachnologists to look through their collections for reliably identified female erigonine specimens that she hasn't yet photographed, and send them to her. Can you help?