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The spider family Thomisidae (crab spiders) has a worldwide distribution and includes 2166 described species, ranking it among the few most species-rich spider families (Platnick 2014). Of this diversity, 130 species are known from North America north of Mexico, with many of these species found in the southwest, although at least one is truly arctic and several others are found in subarctic/alpine tundra. Around half of the North American species are in the genus Xysticus. (Dondale 2005; Bradley 2013).
Some thomisids are associated with flowers whereas others are found on vegetation or in leaf litter or on the ground. Their typically compact, globose bodies with relatively short, thick legs and a crab-like laterigrade posture (i.e., the legs extend sidewise and the femora, especially, are twisted so that the front surface faces up) makes thomisids generally easy to recognize as such. The first pair of legs is often armed with heavy raptorial spines that are used in prey capture. Like most spiders, thomisids have eight eyes. The lateral eyes are typically situated on prominent pale tubercles and are often larger than the median eyes. Flattened thomisids in the genus Coriarachne live in crevices beneath loose treee bark but can also be found on fence posts and wooden buildings. Flower-inhabiting thomisids (Misumena, Minumenoides, Misumenops) ambush pollinating insects. Some of these spiders can slowly change their color to match the background. The ecology of one species, Misumena vatia, has been thoroughly studied by Douglass Morse and colleagues (Morse 2007). Dondale (2005) provides an overview of the taxonomic history of thomisids in Nort America north of Mexico.