Giant Crab (Olios giganteus)

Physical description and identification

Olios giganteus is the large, pale huntsman spider of the Sonoran and Mojave deserts: flat-bodied, long-legged and fast, with laterigrade legs rotated at the base so they extend sideways and the joints hinge horizontally. That posture is where the name comes from — the spider scuttles sideways and backwards as readily as forwards. It is not, however, a true crab spider; those belong to a different family, Thomisidae, and are a fraction of the size.

Adults

• Size: females have a body length of about 18–23 mm and males about 15–20 mm, with a leg span usually between 5 and 7.5 cm (2–3 inches). The frequently repeated figure of a six-inch leg span is an exaggeration; this is a hand-sized spider, and its habit of appearing suddenly on a wall does the rest of the work.
• Colour: pale tan, sand or greyish-yellow, sometimes faintly golden, with a lightly mottled carapace and an abdomen carrying a darker median heart mark. The colour is close to the sand and dead saguaro ribs it hides in.
• Eyes: eight eyes in two rows of four, all of roughly similar size. The claim that this species has "two prominent forward-facing eyes" is wrong — no huntsman has the enlarged front pair of a jumping spider. The eyes do carry a strong reflective layer, so a torch beam at night returns bright points from a spider ten metres away.
• Distinctive features: the best field mark is the chelicerae. The fang bases are dark reddish-brown to near black and contrast sharply with the pale face and carapace. Males are slimmer with longer legs and swollen palp tips; females are broader across the abdomen.

Eggs and the egg sac

The female spins a flattened, papery white sac holding well over a hundred eggs and fixes it under a rock, inside a rolled leaf, under loose bark or in the ribs of a dead saguaro. She then sits over it and guards it, feeding little until the young emerge. Unlike a wolf spider, she never carries the sac on her spinnerets.

Spiderlings

Hatchlings are pale, almost translucent versions of the adult and take their first moult inside or beside the sac before dispersing on foot. They darken and gain their mottling over successive moults, reaching adult size in their second year across most of the range.

No web

Giant crab spiders build no snare whatsoever. Silk goes into the egg sac, into flat retreat sheets under bark or rock, and into draglines — nothing else. Prey is run down or ambushed at close range, which is why the spider is usually seen crossing an open surface rather than sitting in one place.

Where the giant crab spider lives

This is a North American desert species with a fairly tight range, and one widely repeated claim needs correcting: Olios giganteus is not found "around the world in Asia and Australia". That description belongs to the huntsman family as a whole, which is pantropical and contains over a thousand species. The species itself is confined to the southwestern United States and northwestern Mexico.

In the US it is common in Arizona and southern California, and occurs in southern Nevada, southern Utah and New Mexico, reaching west Texas at the eastern edge of its range. South of the border it lives in Sonora, Chihuahua and Baja California.

Its habitat is dry and warm: creosote flats, rocky slopes, washes and desert scrub, with daytime retreats under exfoliating rock, inside the dried rib cavities of dead saguaros, under the loose bark of mesquite and palo verde, and in debris at the base of agaves. It also uses buildings freely — garages, sheds, porches and the walls of lit rooms — which is why a spider of empty desert is one of the most frequently reported house spiders in Phoenix and Tucson.

Are giant crab spiders venomous?

Yes, in the same way every hunting spider is: venom is what subdues a cockroach. It is not medically significant to humans. There is no antivenom for any huntsman spider because none has ever been needed, and this species has no history of serious envenomation. Its venom contains none of the tissue-destroying enzymes that make the recluse spiders a genuine medical problem in the same states, and none of the neurotoxins that make widow spiders dangerous.

The fear the species generates comes almost entirely from size and speed. A pale spider the width of a hand that appears on a bedroom wall at midnight and crosses it in two seconds is alarming, but alarming is not the same as dangerous.

Can giant crab spiders bite?

They can, and occasionally do, but only in defence. The normal response of a giant crab spider to a person is to freeze, then sprint for cover; a cornered one may raise its front legs and hold its ground, and a female guarding an egg sac will stand over it rather than flee. Bites happen when a spider is trapped inside clothing or bedding, or grabbed by hand.

Because the fangs are large, the bite is felt as a sharp jab. The usual result is a pair of small puncture marks, immediate local pain, redness and mild swelling that settle within a few hours to a day. There is no necrosis, no systemic illness and no lasting damage. Wash with soap and water, apply a cold pack and take an ordinary painkiller if needed. Seek medical advice if pain is severe or spreading, if the wound looks infected after a day or two, or if there is any sign of an allergic reaction. Photograph the spider rather than catching it by hand.

Ecological importance and behavior of the giant crab spider

The giant crab spider is a nocturnal cursorial hunter — a spider that finds its food by walking to it. It spends the heat of the day flattened into a crevice behind a slab of rock or inside a saguaro rib, emerges at dusk, and works rock faces, trunks, walls and ceilings through the night. Prey is detected mostly by vibration and by a moving silhouette, then taken with a rush of a few centimetres and a bite that holds.

Diet: cockroaches make up a large share, along with crickets, moths, beetles, grasshoppers, silverfish and other spiders, and large individuals occasionally take small lizards. In a desert house, one resident giant crab spider is more thorough cockroach control than most sprays.

Natural predators: spider-hunting wasps in the family Pompilidae are the specialists, stinging an adult into paralysis and sealing it in a burrow with an egg. Beyond them, the spiders are taken by roadrunners and other birds, by geckos and whiptail lizards, by centipedes and scorpions, and by larger spiders; bats catch wandering individuals off walls.

Prey–predator dynamics: because it hunts on vertical surfaces after dark, it exploits a layer that ground-running wolf spiders and sit-and-wait web builders do not reach. Around desert housing it is one of the few large predators that follows cockroach populations indoors.

Seasonal behaviour: activity peaks through the warm months and falls away in the coolest part of winter. Males mature in spring and early summer and wander to find females, and it is these wandering males that account for most indoor sightings. Total lifespan is around one to two years.

Relationship with humans: beneficial and best tolerated. If one must be removed, put a wide container over it, slide stiff card underneath and release it outside. Sealing gaps under doors and window screens, and cutting back the outdoor lighting that draws the moths it hunts, does far more than pesticide.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The sideways gait is an anatomical consequence rather than a choice: the leg segments are twisted so the joints hinge horizontally, which lets the spider squeeze into gaps a few millimetres deep and reverse out again without turning.
• The "banana spider" nickname sometimes attached to this species belongs to a different huntsman, Heteropoda venatoria, which genuinely does travel in banana shipments. Olios giganteus is a desert native that has never needed a boat.
• Olios is one of the largest huntsman genera in the world, with well over a hundred described species across the warm regions of Africa, Asia, Australia and the Americas — but only a handful live in the United States. Other deserts have their own huntsman specialists, such as the Namib's dancing white lady.
• A female guarding an egg sac will often refuse to abandon it even when the rock she is under is lifted — one of the few times you get a good look at this spider without it running.

In summary, the giant crab spider is a big, pale, fast, harmless desert huntsman of the American Southwest. It builds no web, it is not a crab spider and it is not a recluse — and the cockroach it is chasing across your wall is a better reason to open the door than to reach for a shoe.