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Genus Xysticus - Ground Crab Spiders

Large Crab Spider-any idea which one? - Xysticus elegans Crab Spider - Xysticus elegans  large spider - Xysticus elegans Xysticus - female Crab spider came in for a snack - Xysticus Ground Crab Spider - Xysticus X. alboniger - dorsal - Xysticus alboniger - female Crabby - Xysticus
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Chelicerata (Chelicerates)
Class Arachnida (Arachnids)
Order Araneae (Spiders)
Infraorder Araneomorphae (True Spiders)
No Taxon (Entelegynae)
Family Thomisidae (Crab Spiders)
Genus Xysticus (Ground Crab Spiders)
Synonyms and other taxonomic changes
Explanation of Names
A Latin loan word from Greek.
Refers to the manner in which atheletes would clean themselves before the invention of the shower.(1)
67 species in NA.(1)
Females 5-10mm; males around 4mm.
Xysticus usually has a well defined dorsal stripe on the carapace:

The anterior and posterior lateral eyes are on incompletely conjoined tubercles(1):

Tibia 1 with more than two pairs of ventral macrosetae and femur 1 slender (about four times longer than wide)(1):

Images of palpi and epigyna can be found here.

* 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

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
Widespread in NA.(1)
Generally found on ground or close to it.
Insects, spiders.
Very similar appearance to Coriarachne, Bassaniana, and Ozyptila.
Internet References

~ - Xysticus benefactor female & male from WA.
Works Cited
1.Spiders of North America: An Identification Manual
D. Ubick, P. Paquin, P.E. Cushing and V. Roth (eds). 2005. American Arachnological Society.