Selenopidae

Simon, 1897

Flatties, Wall Crab Spiders, Wall Spiders

Genus Guides

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Selenopidae, commonly known as flatties or wall crab spiders, is a of , free-ranging araneomorph spiders first described by Eugène Simon in 1897. The family contains over 281 in nine , with Selenops being the most well-known. These spiders are distinguished by their extremely flattened dorsoventral profile and laterigrade legs that project horizontally, adaptations that allow them to occupy narrow crevices. They are among the fastest terrestrial animals in terms of turning speed, capable of striking prey in approximately 0.125 seconds through rapid leg-driven spins.

Selenops by (c) Francisco Farriols Sarabia, some rights reserved (CC BY), uploaded by Francisco Farriols Sarabia. Used under a CC-BY license.Selenops insularis by Sarah Crews. Used under a CC BY 3.0 license.Wall crab spider (Selenops insularis) male by Charles J. Sharp . Used under a CC BY-SA 4.0 license.

Pronunciation

How to pronounce Selenopidae: //sɛˈliː.noʊ.pɪˌdiː//

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Identification

Flatties are immediately recognizable by their extremely flattened body profile, appearing as if pressed dorsoventrally. The laterigrade leg orientation—legs projecting horizontally rather than vertically—is diagnostic and shared with only a few other spider . Six of the eight are arranged in a single row across the , with the remaining two set back on either side. The rear edge of the sternum is notched, visible only when the spider is examined ventrally. Legs increase in length from front to back, contrasting with philodromid crab spiders where the second pair is longest. Body length ranges from 7.5–13 mm in North American , with sprawling leg spans making them appear larger. Coloration is variable, including shades of gray, brown, yellow, and orange, often with darker markings and abdominal spots or mottling that provides camouflage against rock or bark surfaces.

Images

Habitat

Selenopids occupy diverse from sea level to over 2,500 meters elevation, primarily in tropical and subtropical regions though some occur in deserts. They are strongly associated with flat surfaces and narrow cracks—commonly found on walls, under rocks, beneath bark on logs or trees, between leaf bases of dense plants, and in cave entrances. Their flattened allows them to exploit narrow crevices inaccessible to most spiders. Several species occur in human habitations.

Distribution

Worldwide distribution with concentration in tropical and subtropical regions. In North America, five Selenops range from southern California through western Texas, plus Florida and the Caribbean. Selenops is the most widely distributed globally. Anyphops occurs throughout Sub-Saharan Africa. The extends from Argentina and Paraguay northward through the Americas to the southwestern United States, with additional representation in Africa, Madagascar, Asia (including China and Southeast Asia), Australia, and the Mediterranean region (including Italy).

Seasonality

activity pattern; spiders emerge at night to hunt and are sedentary during daylight hours when they retreat to crevices. Specific seasonal patterns vary by region and .

Diet

Active that hunt without building webs. Prey capture involves rapid ambush strikes enabled by exceptional turning speed. Specific prey items are not well documented in the provided sources.

Life Cycle

Females construct silk retreats for sacs and guard eggs and spiderlings. approximately one month in mid to late summer for at least some . Developmental details beyond this are not well documented for the as a whole.

Behavior

and free-ranging; do not build capture webs. Among the fastest terrestrial animals in terms of rotational maneuverability, capable of executing leg-driven spins to strike prey in approximately one-eighth of a second. This rapid turning ability, facilitated by their outward leg stance that tracks parallel to the ground, provides unrestricted motion and has attracted robotics research interest. When disturbed, they can sprint with startling speed. Generally sedentary during daylight, hiding in narrow cracks and crevices.

Ecological Role

in rocky, arboreal, and anthropogenic . Their crevice-dwelling habits suggest potential role in controlling insects in narrow spaces inaccessible to other predators.

Human Relevance

Not dangerously venomous to healthy humans. Occasionally encountered in homes, particularly in Florida and the southwestern United States, where they may cause alarm due to unusual appearance and speed. Their rapid turning mechanics have been studied for robotics applications.

Similar Taxa

  • Philodromidae (Running Crab Spiders)Also have laterigrade legs and crab-like appearance, but differ in having the second pair of legs longest (versus increasing length from front to back in Selenopidae) and different arrangement
  • Sparassidae (Giant Crab Spiders/Huntsman Spiders)Share laterigrade legs and flattened bodies, but are generally much larger (some exceeding 40 mm body length), have different arrangements, and lack the extremely flattened profile of selenopids
  • Thomisidae (Crab Spiders)Also called crab spiders and have somewhat flattened bodies with laterigrade legs, but typically have shorter, stouter legs adapted for ambush on flowers rather than rapid running, and different patterns

Sources and further reading