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Myths about Identifying Spiders

Myth: Spiders are easy to identify.

Fact: No such luck! Laypersons often assume that there are only a few spider species around, and all they'd need to identify them would be a few pictures. In reality, the world holds over 50,000 species of spiders classified into over 100 families. In your local area, there are likely at least 30 families and a few hundred species.

Even identifying a spider to family is no trivial task; all the many published keys to spider families are so organized that a beginner will go wrong about half the time. At species level, one needs an expensive microscope, a library of hundreds of separate books, monographs and articles, and a few years of experience to understand the many microscopic details that identify a spider, their similarities, differences, and variation.

Eye arrangements: front view of carapace of 5 genera and 5 families of North American spiders, showing eyes. Warning: these are only examples! There are many other 8-eyed arrangements; then there are the 6-eyed ones like the "recluse" group... Click image to enlarge	End of leg of Cybaeus: trichobothria (arrows), spines, claws (at end).      (from Rod Crawford photo) Click image to enlarge
We tell families of spiders (100 worldwide, perhaps 30 in a given area) apart by characters like those above: eye arrangements, arrangement of trichobothria (special thin sensory hairs), spines, and the claws at the ends of the legs.

Shamrock spider, Araneus trifolium orbweavers, specimens from 1 site.         (from a photo by Bob Thomson) Click image to enlarge	Flower crab spiders, Misumena vatia females & male. (photos:   Bob Thomson, J.W.Thompson Co.) Click image to enlarge	Cross spider orbweavers, Araneus diadematus, 4 different females      (from photos by Bob Thomson) Click image to enlarge
Specimens of the same species can differ radically... in the arrangement of spots (left) or in overall color (right). Some species, like Misumena vatia (center), differ sharply by sex and even the same specimen can change color!

Myth: Spider species are distinguished and identified by "markings."

Fact: No, they're not. Spiders do not come color-coded for our benefit. Imagine trying to identify the make and model of a car...by the color!

Spiders are identified by structure. They are classified into families by the arrangement of the eyes (see above), number of claws, location and arrangement of certain specialized hairs and spines (see above), structure and arrangement of the spinnerets (silk spinning organs at rear end), and other characters that you cannot see with the naked eye. Within families, species are separated mostly by the fine structure of the sex organs (yes, really, I'm not kidding! see below), which can't be seen without high magnification.

Color patterns can be very variable within species, and very similar between different species. For example, the majority of all spider species can be seen as having a "violin" shape somewhere on their bodies; thousands of species have a pattern of "chevrons" on the abdomen. These and other pattern features do not indicate any particular species, and are not signs of danger to humans.

There are exceptions to this rule; a very small number of species do have distinctive pattern elements; but in general, to recognize a spider by naked-eye appearance one must first know all, or almost all, the hundreds of species that live in your locality, their similarities, differences, and variability. Even then, you must usually have a microscope to do more than guess at the spider's identity.

Male Dictyna sublata showing palps from above	Dictyna sublata left palp from below	Dictyna suprenans left palp from below	Dictyna maxima left palp from below	Dictyna zaba left palp from below
Could you identify them? In North America are about 150 species of Dictyna, a genus (split into smaller groups by some) of small spiders. Many, like Dictyna sublata (left), are only about 3 mm long. The whole bodies of most Dictyna look almost exactly alike and the differences are in the male and female sex organs, which are similar but not identical to each other; see male palps of 4 related species above. Could you pick out these 4 species from the 145 other Dictyna? How about without a microscope?

Text © 2003, Burke Museum of Natural History & Culture, University of Washington, Box 353010, Seattle, WA 98195, USA Phone: 206-543-5590 Photos © as credited

Queries to Spider Myths author, Rod Crawford This page last updated 29 August, 2003 This site best viewed at 800 x 600 using IE 5.0 or above.