Thomisidae

Sundevall, 1833

crab spiders, flower spiders, flower crab spiders

Genus Guides

17

Thomisidae is a large of spiders comprising approximately 172 and over 2,100 worldwide. Members are commonly called crab spiders or flower spiders due to their laterigrade leg orientation—legs twisted at the base to project horizontally—enabling sideways and backward movement reminiscent of true crabs. These spiders are ambush that do not construct prey-capture webs, instead relying on stealth, camouflage, and rapid leg strikes to subdue prey. Many species exhibit remarkable color change abilities to match floral backgrounds, while others mimic bird droppings or hunt in leaf litter and bark crevices.

Misumenops by (c) Wynand Uys, some rights reserved (CC BY), uploaded by Wynand Uys. Used under a CC-BY license.Xysticus ferox by (c) Matthew Lindsey, some rights reserved (CC BY), uploaded by Matthew Lindsey. Used under a CC-BY license.Xysticus ferox by (c) Justin Williams, some rights reserved (CC BY), uploaded by Justin Williams. Used under a CC-BY license.

Pronunciation

How to pronounce Thomisidae: //θoʊˈmɪsɪdiː//

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Identification

Thomisids are distinguished from other spider by their laterigrade legs and enlarged leg pairs held in a crab-like posture. They may be confused with Sparassidae (giant crab spiders) and Philodromidae (running crab spiders), which share the laterigrade leg condition; however, thomisids generally have more robust, spiny front legs and a more compact body. Huntsman spiders (Sparassidae) are typically larger with longer, more slender legs. Philodromids lack the extreme foreleg enlargement and spination of thomisids. Some thomisids resemble bird-dropping spiders of Araneidae, but differ in leg structure and arrangement. The ocular quadrangle pattern varies but is not diagnostic at the family level.

Images

Appearance

Thomisids possess a distinctive body plan characterized by laterigrade legs: the leg bases are twisted so that the legs extend horizontally rather than vertically, allowing the spider to scuttle sideways and backward. The first two pairs of legs are markedly longer, thicker, and more robust than the rear pairs, armed with strong spines for seizing prey. The body is typically somewhat flattened. Coloration ranges from cryptic browns and grays to vivid greens, pinks, whites, or yellows; some can change color over days to match their substrate. The rear legs are smaller and covered with prominent spines. Sexual size dimorphism is pronounced in many species, with females often substantially larger than males.

Habitat

Thomisids occupy diverse microhabitats. Many are associated with flowers, where they ambush pollinating insects. Others hunt on leaves, bark, or in crevices of tree trunks. Some , such as Xysticus, are ground-dwelling and hunt in leaf litter. Species with flattened bodies exploit narrow cracks under loose bark or in wood. Certain species are excellent mimics of bird droppings and sit exposed on leaves. The shows broad ecological flexibility across terrestrial .

Distribution

distribution with recorded on every continent except Antarctica. Highest diversity in tropical and subtropical regions. The is well-represented in North America, Eurasia, Africa, Australia, and throughout the Neotropics. Specific regional studies document established faunas in New Zealand, India, Argentina, and the southwestern United States.

Seasonality

Activity patterns vary by and latitude. In temperate regions, are typically active during spring and summer. Some species overwinter as immatures; for example, male Thomisus labefactus emerging in September mostly overwinter before molting to adulthood in May, while females overwinter once and reach adulthood in June. In subtropical and tropical regions, activity may extend throughout the year.

Diet

Strictly predatory, feeding on insects and other arthropods. Flower-dwelling capture including bees, butterflies, and flies. Some species regularly take large, well-defended prey such as honey bees and arctic butterflies. The diet of ground-dwelling species includes various ground-active insects. Males of some species may supplement their diet with flower pollen when unable to secure large prey items.

Life Cycle

Development includes , postembryonic instars, and stages. Females produce egg sacs, often wrapped in folded leaves or other silk retreats, and guard them until spiderlings emerge. In Misumena vatia, females do not feed during the guarding period. Instar number varies: Thomisus labefactus males typically 3 times after to reach adulthood, females molt once before overwintering and 5 times after. Misumenops pallida shows increasing instar length and feeding rate throughout development, with elevated mortality in third and fourth instars. Lifespan averages approximately 151 days for females and 160 days for males in studied .

Behavior

Ambush is the primary foraging strategy; thomisids do not build prey-capture webs though they produce silk for draglines and . Some are sit-and-wait on flowers, while others actively wander. Color change —transitioning between white and yellow phases to match flowers—has evolved once within the and occurs in species such as Misumena vatia and Thomisus spectabilis, taking 1-20 days to complete. Males of some species exhibit pre-copulatory guarding, mounting subadult females before their final to secure mating access immediately after . Some species are mimics: Aphantochilus species mimic and prey upon Cephalotes ants.

Ecological Role

Significant of insects in multiple . Flower-dwelling influence and potentially plant reproductive success through pollinator . In agricultural systems, species such as Misumenops pallida serve as natural enemies of insect pests in soybean fields. Their ambush strategy fills a distinct predatory from web-building spiders.

Human Relevance

Generally harmless to humans; not known to possess medically significant venom. Valued in of agricultural pests. Subject of considerable scientific interest due to color change abilities, , and predatory . Popular subjects for nature photography and macrophotography. Occasionally confused with dangerously venomous Sicarius , which are sometimes colloquially called 'six-eyed crab spiders' but belong to a distantly related .

Similar Taxa

Misconceptions

The 'crab spider' is applied to multiple unrelated . Some members of Sparassidae, Selenopidae, and Sicariidae are also called crab spiders. Sicarius , sometimes called 'six-eyed crab spiders,' are highly venomous but unrelated to Thomisidae; this has caused confusion about the medical significance of true crab spiders.

More Details

Sexual dimorphism

Extreme size differences between sexes are documented, with female Thomisus onustus averaging more than 60 times the mass of males. This dimorphism has been explained by the hypothesis (larger females produce more ) and the gravity hypothesis (smaller males travel more efficiently to find mates). Males of some mature faster and wander constantly seeking females.

Phylogenetic insights

Molecular studies of New Zealand thomisids suggest the split from Australian relatives approximately 5-6 million years ago, with and subsequent radiation explaining current diversity patterns. Higher-level phylogenetic analyses indicate that many thomisid were historically defined by plesiomorphic characters, and that arrangement patterns have evolved convergently multiple times.

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Sources and further reading