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The Thomisidae are a
family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideall ...
of
spider Spiders ( order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species div ...
s, including about 170
genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomencl ...
and over 2,100 species. The common name crab spider is often linked to species in this family, but is also applied loosely to many other families of spiders. Many members of this family are also known as flower spiders or flower crab spiders.


Description

Members of this family of spiders do not spin webs, and are ambush predators. The two front legs are usually longer and more robust than the rest of the legs. The back two legs are smaller, and are usually covered in a series of strong spines. They have dull colorations such as brown, grey, or very bright green, pink, white or yellow. They gain their name from the shape of their body, and they usually move sideways or backwards. These spiders are quite easy to identify and can very rarely be confused with
Sparassidae Huntsman spiders, members of the family Sparassidae (formerly Heteropodidae), are known by this name because of their speed and mode of hunting. They are also called giant crab spiders because of their size and appearance. Larger species sometim ...
family, though the crab spiders are usually smaller.


Etymology

Spiders in this family are called "crab spiders" due to their resemblance to
crabs Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting " tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all th ...
, the way such spiders hold their two front pairs of legs, and their ability to scuttle sideways or backwards. The Thomisidae are the family most generally referred to as "crab spiders", though some members of the
Sparassidae Huntsman spiders, members of the family Sparassidae (formerly Heteropodidae), are known by this name because of their speed and mode of hunting. They are also called giant crab spiders because of their size and appearance. Larger species sometim ...
are called "giant crab spiders", the Selenopidae are called "wall crab spiders", and various members of the
Sicariidae Sicariidae is a family of six-eyed venomous spiders known for their potentially necrotic bites. The family consists of three genera and about 160 species. Well known spiders in this family include the brown recluse spider and the six-eyed sand ...
are sometimes called "six-eyed crab spiders". Some distantly related orb-weaver spider species such as ''
Gasteracantha cancriformis ''Gasteracantha cancriformis'' (spinybacked orbweaver) is a species of orb-weaver spider (family Araneidae). It is widely distributed in the New World. The genus name ''Gasteracantha'' derives from the Greek words (', "belly") and (', "thorn" ...
'' also are sometimes called "crab spiders".


Behavior

Thomisidae do not build webs to trap prey, though all of them produce
silk Silk is a natural protein fiber, some forms of which can be woven into textiles. The protein fiber of silk is composed mainly of fibroin and is produced by certain insect larvae to form cocoons. The best-known silk is obtained from the co ...
for drop lines and sundry reproductive purposes; some are wandering hunters and the most widely known are
ambush predator Ambush predators or sit-and-wait predators are carnivorous animals that capture or trap prey via stealth, luring or by (typically instinctive) strategies utilizing an element of surprise. Unlike pursuit predators, who chase to capture pre ...
s. Some species sit on or beside flowers or fruit, where they grab visiting insects. Individuals of some species, such as ''
Misumena vatia ''Misumena vatia'' is a species of crab spider with a holarctic distribution. In North America, it is called the goldenrod crab spider or flower (crab) spider,Acorn, John and Sheldon, Ian. (2003). ''Bugs of Ontario'' Edmonton: Lone Pine Publishi ...
'' and '' Thomisus spectabilis'', are able to change color over a period of some days, to match the flower on which they are sitting. Some species frequent promising positions among leaves or bark, where they await prey, and some of them sit in the open, where they are startlingly good mimics of bird droppings. However, these members of the family Thomisidae are not to be confused with the spiders that generally are called bird-dropping spiders, not all of which are close relatives of crab spiders. Other species of crab spiders with flattened bodies either hunt in the crevices of tree trunks or under loose bark, or shelter under such crevices by day, and come out at night to hunt. Members of the genus ''
Xysticus ''Xysticus'' is a genus of ground crab spiders described by C. L. Koch in 1835, belonging to the order Araneae, family Thomisidae. The genus name is derived from the Ancient Greek root ''xyst'', meaning "scraped, scraper". Description ''Xysticu ...
'' hunt in the leaf litter on the ground. In each case, crab spiders use their powerful front legs to grab and hold on to prey while paralysing it with a venomous bite. The spider family Aphantochilidae was incorporated into the Thomisidae in the late 1980s. ''
Aphantochilus ''Aphantochilus'' is a genus of ant-mimicking crab spiders that was first described by Octavius Pickard-Cambridge in 1871. it contains three species, found in Paraguay, Brazil, Argentina, and Panama: '' A. cambridgei'', '' A. inermipes'', and ...
'' species mimic ''
Cephalotes ''Cephalotes'' is a genus of tree-dwelling ant species from the Americas, commonly known as turtle ants. All appear to be gliding ants, with the ability to "parachute" and steer their fall so as to land back on the tree trunk rather than fall ...
'' ants, on which they prey. The spiders of Thomisidae are not known to be harmful to humans. However, spiders of a distantly related genus, '' Sicarius'', which are sometimes referred to as "crab spiders", or "six-eyed crab spiders", are close cousins to the
recluse spider The recluse spiders (''Loxosceles'' (), also known as brown spiders, fiddle-backs, violin spiders, and reapers, is a genus of spiders that was first described by R. T. Lowe in 1832. They are venomous spiders known for their bite, which sometimes ...
s, and are highly
venomous Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a sti ...
, though human bites are rare.


Sexual dimorphism

Several different types of
sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most anim ...
have been recorded in crab spiders. Some species exhibit color dimorphisms; however, the most apparent dimorphism is the difference in size between males and females. In some species, this is relatively small; females of ''
Misumena vatia ''Misumena vatia'' is a species of crab spider with a holarctic distribution. In North America, it is called the goldenrod crab spider or flower (crab) spider,Acorn, John and Sheldon, Ian. (2003). ''Bugs of Ontario'' Edmonton: Lone Pine Publishi ...
'' are roughly twice the size of their male counterparts. In other cases, the difference is extreme; on average, female ''
Thomisus onustus ''Thomisus onustus'' is a crab spider belonging to the genus '' Thomisus''. These spiders are found across Europe, North Africa, and parts of the Middle East and Asia. ''T. onustus'' reside in flowers in lowland vegetation. Females are distinguis ...
'' are more than 60 times as massive as the males. Several hypothesized explanations are given for the evolution of sexual size dimorphisms in the Thomisidae and other sister taxa. The most widely acknowledged hypothesis for female growth is the
fecundity Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to p ...
hypothesis: selection favors larger females so they can produce more eggs and healthier offspring. Because males do not carry and lay eggs, a growth in size does not confer a fitness advantage. However, sexual size dimorphism may be a result of male dwarfism. The gravity hypothesis states that the smaller size allows the male to travel with greater ease, providing him with an increased opportunity to find mates. Females are comparatively stationary, and their larger size allows them to capture larger prey, such as butterflies and bees, granting females the additional nutrients necessary for egg production. Other hypotheses propose that sexual size dimorphism evolved by chance, and no selective advantage exists to larger females or smaller males.


Taxonomy

, this large family contains around 171
genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomencl ...
:


See also

*
Philodromidae Philodromidae, also known as philodromid crab spiders and running crab spiders, is a family of araneomorph spiders first described by Tord Tamerlan Teodor Thorell in 1870 (then known as subfamily Philodrominae within Thomisidae). It contains o ...
, also called crab spiders


References


External links


Pictures of crab spiders
(free for noncommercial use)
Photos of crab spiders
hosted by University of California, Berkeley {{Taxonbar, from=Q12023 Araneomorphae families Taxa named by Carl Jakob Sundevall