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Genus Coriarachne

crab spider on door - Coriarachne brunneipes dark spider on tree - Coriarachne brunneipes black crab spider - Coriarachne brunneipes - female Crab spider - Coriarachne brunneipes - female Crab spider - Coriarachne brunneipes - female Crab Spider - Coriarachne brunneipes - female ?Coriarachne brunneipes - Coriarachne brunneipes Bark spider? - Coriarachne brunneipes
Classification
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Chelicerata (Chelicerates)
Class Arachnida (Arachnids)
Order Araneae (Spiders)
Infraorder Araneomorphae (True Spiders)
No Taxon (Entelegynes )
Family Thomisidae (Crab Spiders)
Genus Coriarachne
Numbers
1 Species in the US & Canada.

The 3 other species listed below were part of the genus previously, but have now been moved to Bassaniana.
Identification
Spiders of the genus Coriarachne are crablike, with flattened bodies and long laterigrade legs. The colors are sombre and blend in with the tree bark, fence posts, and wooden buildings on which they are found. (1)

Coriarachne can be separated from other genera by the accentuated flatness of the carapace and by the anterior eye row being either straight or slightly recurved...and lacking the pale band on the carapace. (2)

* Ozyptila and Xysticus are both distinguished from either Coriarachne or Bassaniana by the lateral profile of the carapace: as Lynette mentioned, in both Coriarachne and Bassaniana the carapace is roughly flat all the way from front to back. The side profile of the carapace for Ozyptila and Xysticus, on the other hand, will be higher near the front and will drop down near the back at around coxa three. Coriarachne and Bassaniana maintain roughly the same level height from front to back.

So, the order of things to check when separating Xysticus, Ozyptila, Coriarachne and Bassaniana is:

1) Check carapace flatness first, to separate Coriarachne and Bassaniana from Ozyptila or Xysticus

2) Then, check the ventral macrosetae on tibia I and femur I length/width ratio to separate Xysticus from Ozyptila.

Ozyptila: - no more than 2 pairs of ventral macrosetae on tibia I .... femur I length-to-width ratio is about 3:1

Xysticus: - 3 to 4 pairs of ventral macrosetae on tibia I .... femur I length-to-width ratio is about 4:1

As for separating Coriarachne from Bassaniana, Lynette knows more about that than I do. :)

Anyway, that's how I've been separating these genera, using the information from Spiders of North America, Ubick et al. (2005) page 247. ~ John Sloan 2 Aug. 2009

From Rod Crawford: "This is either Coriarachne brunneipes or Bassaniana utahensis. The key feature is whether the large setae on the carapace have pointed ends (Coriarachne) or blunt ends (Bassaniana)"
Range
C. brunneipes - Western United States from Rocky Mountains to Pacific Coast in Canada from British Columbia eastward to northern Ontario.

B. floridana - Southeastern United States extending northward into New England and southern Ohio. (2)

B. versicolor - Eastern United States and southern Ontario westward to the Rocky Mountains, in the Southwest as far as western Arizona. (2)

B. utahensis - Trans-Canadian and northern United States, along Gulf coast states from Florida into Mexico, also from central Alaska south along mountain ranges into Mexico. (2)

(There is crossbreeding between utahensis & versicolor where their range overlaps)
See Also
Bassaniana utahensis
Works Cited
1.The Crab Spiders of Canada and Alaska; Araneae: Philodromidae and Thomisidae (The Insects and Arachnids of Canada, Part 5)
Charles Dondale & James Redner. 1978. Research Branch, Agriculture Canada, Publ. 1663: 1-255.
2.A Taxonomic Revision of the Crab Spider Genus Coriarachne for North America North of Mexico.
T. A. Bowling and R. J. Sauer. 1975. J . Arachnol. 2 :183-193 .