Myth: You identify spiders by "markings"

October 20, 2015
Rod Crawford

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, number of claws, location and arrangement of certain specialized hairs and spines, 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.

 

Dramatic variation

Specimens of the same species can differ radically...in the arrangement of spots (see the shamrock spiders below) or in overall color (like the orbweavers). Some species, like Misumena vatia (flower crab spiders), differ sharply by sex and even the same specimen can change color!

Shamrock spider specimens

Shamrock spider, Araneus trifolium orbweavers, specimens from one site.
Photo: Bob Thomson

 

 

Flower crab spiders

Flower crab spiders, Misumena vatia, females & male.
Top left photo: J.W. Thompson Co. Bottom left and right photos: Bob Thomson

Cross spider orbweavers

Cross spider orbweavers, Araneus diadematus, 4 different females.
Photos: Bob Thomson

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.

 

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 (below, upper 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 5 related species below. Could you pick out these 5 species from the 144 other Dictyna? How about without a microscope?

Male Dictyna sublata showing palps from above.

Male Dictyna sublata showing palps from above. 

Dictyna sublata left palp from below

Dictyna sublata left palp from below.

Dictyna suprenans left palp from below.

Dictyna suprenans left palp from below. 

Dictyna maxima left palp from below.

Dictyna maxima left palp from below. 

Dictyna zaba left palp from below.

Dictyna zaba left palp from below. 

Dictyna serena left palp from below.

Dictyna serena left palp from below.

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