Friday, September 18, 2015

Spiders Cannot Be Identified By Color And Pattern Alone

Atypical Araneus diadematus
female photographed in
Seattle, Washington, USA
Typical appearance of
Araneus diadematus female.
Photo by Friedrich Böhringer
My neighborhood is absolutely festooned with the introduced orb-weaver Araneus diadematus (Araneidae).  By late summer, hardly a shrub or a structure is devoid of one of these orange and brown spiders with the striking white pattern on the dorsum (left).  So I got excited when I spotted a large orange orb-weaver (right) on my balcony that lacked the color and patterning of A. diadematus.  Could a native species of orb-weaver actually be thriving in Seattle, at my house?!

Answer: No, not in this case.

Genitalia of an atypically colored and patterned
Araneus diadematus female from
Seattle, Washington USA.
The spider in question is fairly large (over 0.5 inch long) with distinctive external genitalia, so I was able to identify her with reasonable certainty from a photo (right) as an A. diadematus. (Most species are much too small and/or have genitalia too complex to be identified via photos.)  Despite this spider's exotic appearance, she was just "more of same", albeit atypically colored and patterned.

So, as nice as it would be to identify spiders in the field from easily observable traits like color and pattern, such features aren't necessarily diagnostic and in fact can be misleading.  The only way to identify spiders with certainty is by examining the shape of their genitalia and other tiny, immutable features under a microscope.

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