The Linyphiidae are some of our most common and least-known spiders. Although they include far more species than any other spider family in North America, these spiders are poorly studied because of their small size and similarities in appearance.
The big subfamily Erigoninae accounts for the bulk of linyphiid diversity, with about 650 species in North America. These tiny spiders are particularly difficult to tell apart. Some are less than a millimeter long, and barely visible with the naked eye. Even under a microscope they often look almost identical.
While the males of these minuscule animals can have striking head modifications and other showy features, females tend to be outwardly drab and undistinguished. Identification relies almost entirely on the details of the complex, scelerotized genital structures, called the epigyna. But illustrations of the female anatomy in the taxonomic literature have tended to be perfunctory for the Erigoninae, and sometimes non-existent. Most genera await a modern revision. As a result, definite determinations can be difficult even for professional arachnologists. Female erigonines are the only spiders in North America for which no key to genus exists, making identification of museum specimens almost impossible in many cases.
LinEpig is an attempt to overcome this problem at the practical level. By making available an online reference to the anatomical structures needed for identification, this Field Museum initiative helps research collections around the country reduce their intractable backlog of linyphiid specimens.
Using digital microscopy tools, we are photographing the external genitalia of female erigonines that have been identified by experts and posting the images in an online gallery for easy reference. All photographs are labeled with the taxonomic details of the species. The Google photo-sharing site provides georeferencing and blog-like commenting functionality that would allow taxonomists to discuss online questions of identification.
Imaging is performed with a stereoscopic microscope. Specimens are immersed in alcohol and immobilized in fine sand or electrophoresis beads. Because of the small size of the specimens, the liquid is chilled to minimize shifting due to Brownian motion as the alcohol absorbs heat from the microscope's light source. The approach takes advantage of the relatively two-dimensional epigynal plates typical of this group to rapidly provide a "ready reference" adequate, in most cases, for distinguishing among closely related species. Additional shots are taken across a depth-of-field range to allow substitution of higher-quality composite images at a later date. A full-body, or "habitus," image is also taken. These were hosted by the now-discontinued Nearctic Spider Database as a further aid in identification, and will be added to this site after re-processing. Collection data is preserved in a Darwin Core 2 data set as used by GBIF.
About the Linyphiidae
The family Linyphiidae occurs worldwide. Unlike most arthropods — indeed, most terrestrial forms of life — they have their greatest diversity not in the tropics, but in the temperate zones. The family accounts for about a quarter of all spider species in Canada and the United States — more than the next three families combined (the Salticidae, Dictynidae and Lycosidae). Living in the leaf litter and grasslands, linyphiids have evolved remarkable variations in form and habits. Commonly called sheet-web spiders or hammock-weavers, they are also known for their "ballooning" method of dispersal, by extruding a strand of silk to catch rising thermals on warm days. They are vigorous agricultural predators and form an important natural check on many crop pests. A linyphiid is the oldest known spider preserved in amber, and linyphiids were among the species described by Carl Alexander Clerck in the 1750s, predating Linnaean taxonomy.
But despite their importance and venerability, they have not had a proportionate degree of attention by contemporary systematists. Much of the literature consists of original descriptions. Within the Erigoninae, a quarter of the nearctic genera are monotypic (known from only one species), and a number of others known from only two species. Not surprisingly, most museums are forced to put them to the side unsorted. It is hoped that by making identification more accessible, work on the Linyphiidae can be facilitated.