LinEpig

About 3,800 species of spiders are known to exist in North America. These species fall into 68 different families, but some of the families contain many more species than others. The Sheetweb Weavers (family Linyphiidae – pronounced "lin-uh-FEE-eh-dee") is a very large family of mostly very tiny spiders. In some parts of the country, a quarter of all the spider species may be Sheetweb Weavers. Many of them are only a couple of millimeters long, or less. And a lot of them look alike – particularly the Dwarf Spiders (subfamily Erigoninae). In fact, the female Dwarf Spiders are actually the only spiders in North America with no key to genus, which means they can't be identified using the standard spider identification guide, Spiders of North America.

LinEpig is an online image gallery designed to make it easier to identify Dwarf Spider females. The name (which we pronounce "LIN-eh-pidge") is a combination of Linyphiidae and "epigynum" – the term for a female spider's reproductive organs. In spiders, the reproductive anatomy is very intricate, and scientists use the details to tell related species apart.

Working with specimens that have been identified by experts, we are using a digital microscope to photograph these anatomical structures, and posting the images in an online gallery for easy reference. By making this online reference available, this Field Museum project helps research collections around the country identify the interesting and diverse linyphiid specimens in their collections.

The Dwarf Spider photos above are by Tom Murray and Kevin Lentz.

Use the LinEpig ID gallery

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 often present an outwardly drab and undistinguished appearance. 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.

The project

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 backlogs 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.

Imaging is performed with a stereoscopic microscope. 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. A full-body, or "habitus," image is also taken. Collection data is preserved in a Darwin Core 2 data set as used by GBIF.

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.

In creating LinEpig, The Field Museum utilizes existing equipment and the unpaid labor of associates familiar with the group. To date, more than 290 species (of the roughly 650 described species of nearctic erigonines) have been incorporated, with the assistance of loans of specimen material from several museums and private collections, as well as contributed images from colleagues working with this group.

Completion will require the participation of a number of other linyphiid-rich collections. We are grateful for either loans of specimens or contributions of good-quality microscopic images. While the main focus of the project is on the female epigynum, we have been imaging habitus of specimens on loan, as well as habitus and palps of any males that happen to be included with the female, in the interests of further aiding identification.

Field Museum images are made available for study and reuse under a Creative Commons license; see details. Images by external contributors are fully credited and, while we encourage people to adopt the "CC-BY-NC" license, all displayed permissions are creator-specific and can be set up to meet your individual needs.

Collection managers with reliably identified erigonine females that are not yet represented at LinEpig who would like to assist with this effort can contact the project manager (below). Images of habitus, male palps, or taxonomically relevant anatomical details are also welcome.

Attached is the list of species we are looking for. If you have females of any of these species—or even better, epigynal images!—please get in touch!

Nina Sandlin (nsandlin@fieldmuseum.org)