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.
Thomisidae Sundevall, 1833
Thomisidae Sundevall, 1833
There are over 2,000 species of crab spiders and they are found all over the world. In North America there are over 200 species.
Biogeographic Regions: nearctic (Native ); palearctic (Native ); oriental (Native ); ethiopian (Native ); neotropical (Native ); australian (Native )
Crab Spiders usually have short, wide, flat bodies. The first two pairs of legs are larger than the back legs, and are usually held open so that the spider can easily grab its prey. They usually walk sideways or backwards, and use just their back legs. This is where they get their name.
All crab spiders have eight eyes, and the eyes on the edges of their cephalothorax are often raised up on bumps, so they can see in all directions. These spiders have small fangs compared to other spiders, but their venom acts quickly to paralyze their prey.
In some species males and females are different colors, and males are often much smaller than females. Crab spiders usually are colored to match their habitat. Some species can slowly (over a period of days) change color to match the color of the flowers they are hiding on.
Other Physical Features: bilateral symmetry
Sexual Dimorphism: sexes alike; female larger
Spiders in this group can be found in just about any habitat where they can hide, adn there are insects to eat. The only places they can't live are the dryest deserts or the coldest mountaintops.
Habitat Regions: temperate ; tropical ; terrestrial
Terrestrial Biomes: taiga ; desert or dune ; chaparral ; forest ; rainforest ; scrub forest ; mountains
Wetlands: marsh ; swamp
Crab spiders ambush their prey, mainly Insecta, sometimes holding still and relying on their camouflage to keep them from being seen by their prey, sometimes running quickly. When a prey animal is close enough, they grab it with their strong front legs and bite it quickly.
Crab spiders' main defense is their camouflage. They will hide or drop away from predators if they can. They can bite other invertebrates, but this doesn't work against vertebrate predators.
Animal / predator / stocks nest with
female of Dipogon bifasciatus stocks nest with Thomisidae
Life History and Behavior
Communication and Perception
We don't know very much about how these spiders communicate. They probably use sight, touch, and smell.
Females lay eggs. The spiderlings that hatch out look like mini-adults. As they grow they have to shed their skins, but they do not change their general shape.
In cold climates the adults of this species usually die when winter comes, so probably live one year or less. In tropical climates they may live longer.
We don't know very much about reproduction in this group.
Breeding season: Varies between species, not during cold months.
Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; sexual ; fertilization (Internal ); oviparous
Females protect their eggs by putting them in a sack made of silk and guarding them.
Parental Investment: female parental care
Molecular Biology and Genetics
Statistics of barcoding coverage
Specimens with Sequences:3056
Specimens with Barcodes:3009
Species With Barcodes:223
No crab spiders are known to be endangered, but many species are still not known to science, and could be disappearing without anyone knowing.
Relevance to Humans and Ecosystems
Economic Importance for Humans: Negative
These spiders can bite, but are not aggressive, and are not dangerous to people. They sometimes eat beneficial insects like honeybees that pollinate crops, but they eat enough pests that this is not usually a problem.
Negative Impacts: injures humans (bites or stings)
Economic Importance for Humans: Positive
Crab spiders eat lots of insects and mites that are pests. They are often a big help to farmers, because they hunt on plants and eat the invertebrates they find there.
Positive Impacts: controls pest population
Crab spider is a common name applied loosely to many species of spiders, but most nearly consistently to members of the family Thomisidae. Among the Thomisidae it refers most often to the familiar species of "flower crab spiders", though not all members of the family are limited to ambush hunting in flowers.
Crab spider as a name in common use
Rationalisation for the name crab spider is generally subjective and anecdotal. It is commonly said to refer to a fancied resemblance to crabs, or to the way such spiders hold their two front pairs of legs, or their ability to scuttle sideways or backwards. Some spiders so called have bodies that are flattened and angular. At all events, the Thomisidae are the family most generally referred to as "crab spiders". However, some members of the Sparassidae are called giant crab spiders, Selenopidae are called wall crab spiders, and various members of the Sicariidae are sometimes called six-eyed crab spiders. Some unrelated Orb-weaver spider species such as Gasteracantha cancriformis also are commonly called "crab spiders."
Such names are of little biological significance, and in this article the emphasis is on the Thomisidae.
Crab spider biology
Thomisidae do not build webs to trap prey, though all of them produce silk for drop lines and sundry reproductive purposes; some are wandering hunters and the most widely known are ambush predators. Some species sit on or beside flowers or fruit, where they grab visiting insects. Individuals of some species, such as Misumena vatia, 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 will sit in the open, where they are startlingly good mimics of bird droppings. However, note that 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 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 paralyzing it with a venomous bite.
The spider family Aphantochilidae was incorporated into the Thomisidae in the late 1980s. Aphantochilus species mimic Cephalotes ants, on which they prey.
The spiders of Thomisidae are not known to be harmful to humans. However, spiders of an unrelated genus, Sicarius, which are sometimes referred to as "crab spiders", or "six-eyed crab spiders", are close cousins to the recluse spiders, and are highly venomous, though human bites are rare.
The following subfamilies are recognized:
- Aphantochilinae (3 genera)
- Bominae Ono, 1984 (9 genera)
- Dietinae (32 genera)
- Stephanopinae (35 genera)
- Stiphropodinae (3 genera)
- Strophiinae (8 genera)
- Thomisinae (67 genera)
- Incertae sedis
- Ansiea Lehtinen,
- Carcinarachne Schmidt, 1956
- Cozyptila Lehtinen & Marusik, 2005
- Ebelingia Lehtinen, 2005
- Facundia Petrunkevitch, 1942 † (fossil)
- Fiducia Petrunkevitch, 1942 † (fossil)
- Henriksenia Lehtinen, 2005
- Hexommulocymus Caporiacco, 1955
- Ledouxia Lehtinen, 2005
- Mastira Thorell, 1891
- Megapyge Caporiacco, 1947
- Modysticus Gertsch, 1953
- Rejanellus Lise, 2005
- Syphax Koch & Berendt, 1854 † (fossil)
- Tarrocanus Simon, 1895
- Taypaliito Barrion & Litsinger, 1995
Female Crab Spider Genus Thomisus Family Thomisidae, sitting in ambush in a Felicia flower.
A typical female Crab Spider in the genus Thomisus.
Thomisus sp. Male feeding on small insect, probably a Heteroptera. The fingerprint gives some idea of the scale.
Crab Spider female, family Thomisidae, genus Synema. Awaiting prey on Lavandula.
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