Red-legged Purseweb (Sphodros rufipes)

Physical description and identification

The red-legged purseweb spider is a mygalomorph, from the same ancient branch of the spider tree as tarantulas and trapdoor spiders, and it is built accordingly: a heavy cylindrical body, a low glossy carapace, and chelicerae so large they jut forward from the head like blunt tusks, approaching a third of the carapace length in a big female.

Adults

• Size: females reach roughly 19-25 mm (0.75-1 in) in body length and are distinctly bulky. Males are 10-15 mm, slimmer, with proportionally longer legs and swollen pedipalp tips.
• Colour: the male is the one everybody photographs, a lacquered black body on rust-red to orange-red legs, the colour strongest on the femora. Females are dark brown to purplish black with dull reddish-brown legs.
• Eyes: eight small eyes packed onto a low mound at the very front of the carapace. Vision is poor and the spider works by touch inside its tube; see our guide to spider eyes.
• Distinctive features: the fangs are paraxial, swinging straight down and back in parallel rather than pinching together as in most spiders. Three tarsal claws, four book lungs and short spinnerets complete the mygalomorph package.

Eggs and egg sac

Mating takes place inside the female tube and the eggs follow weeks later, wrapped in a lens-shaped white sac slung in the buried section where humidity barely shifts. Clutch size in the genus runs to roughly 70-100 eggs, and because the sac never leaves the sealed tube, little is published on this species reproduction.

Spiderlings

Hatchlings are pale, and many young red-legged pursewebs then show the reverse of the adult scheme: a reddish body with a darker head. They stay in their mother tube through the first moults, for months rather than days, and disperse in spring on foot and by short ballooning flights. Growth is slow, and a female needs several years of moulting to reach full size.

The web

The purse web is a sealed silk sock with the spider living inside it, not a snare. A tube of tough silk runs from a burrow in the soil straight up a tree trunk, usually with 15-25 cm (6-10 in) exposed, plastered outside with soil, bark flakes and lichen until it reads as a root or a raised seam of bark. When an insect walks onto the silk the spider drives its fangs through the fabric from inside, slits the tube, drags the prey in and stitches the tear shut. Nothing about it resembles the sheets and orbs in our guide to spider webs.

Where the red-legged purseweb spider lives

Sphodros rufipes is a spider of the southeastern United States. Records cluster across the Gulf and southern Atlantic states, including Florida, Georgia, Alabama, Mississippi, Louisiana and eastern Texas, and reach north through the Carolinas, Tennessee, Kentucky, Arkansas and Missouri to Indiana, with scattered coastal-plain records as far as New Jersey.

One claim in older summaries needs correcting: this is not a Canadian spider. The purseweb that reaches Canada is Sphodros niger, in southern Ontario. S. rufipes has never been established that far north, and it is a United States endemic rather than a spider shared with Mexico.

The habitat requirement is narrow: mature hardwood or mixed woodland with well-drained, diggable soil and standing trunks to build against, from oak and pine woods and river bluffs to the shaded margins of old parks and long-settled suburbs. Strip the litter, compact the ground or fell the veteran trees and the spider vanishes locally, because slow-growing mygalomorphs recolonise very poorly.

Are red-legged purseweb spiders venomous?

Yes. Like every member of Atypidae, the red-legged purseweb spider is venomous and uses that venom to kill the hard-bodied beetles, crickets and ants that blunder onto its tube. For people it is not medically significant: no antivenom exists for any purseweb spider because none has ever been required, and no serious envenomation has been documented anywhere in the genus.

Being a mygalomorph does not put this spider in the category of the Sydney funnel-web or the other Australian funnel-webs, which carry delta-hexatoxins acting on vertebrate nerve channels. Purseweb venom has no comparable component and no recorded systemic effect in humans. Relatedness to a dangerous species tells you nothing about venom potency.

Can red-legged purseweb spiders bite?

They can, and they almost never do. A spider sealed inside a silk tube has little opportunity to meet a person, and a wandering male confronted with a hand freezes or runs rather than rearing up. Practically every recorded bite involves a spider that was picked up, trodden on or trapped against skin.

When a bite does occur, the striking part of it is mechanical rather than chemical. The long paraxial fangs puncture cleanly, so a bite from a large female is described as a sharp, deep pinprick rather than a sting, leaving two marks a few millimetres apart with local soreness and slight swelling that settle within a day. Wash the site with soap and water, apply a cold pack and keep the punctures clean. Seek medical advice if pain increases after the first day or the area becomes red, hot and spreading, which points to infection rather than venom. Do not apply heat, cut the wound or attempt suction.

Ecological importance and behavior of the red-legged purseweb spider

Everything in this spider life is organised around a single silk tube. It digs a burrow, lines it, extends the lining up a trunk and stays, often for years. The prey walks to the spider: ants, beetles, crickets, caterpillars, millipedes and woodlice crossing the tube set the wall vibrating, and the strike comes from inside.

Natural predators: birds, shrews and other small mammals take wandering males, and spider wasps of the family Pompilidae hunt mygalomorphs in their burrows, paralysing the spider as living food for a single larva. Parasitic flies and nematodes are recorded from related atypids. For the resident spider the camouflaged tube is the defence, and its effectiveness is why these spiders are so rarely found.

Prey-predator dynamics: the tube occupies a surface few other predators patrol, the vertical face of a trunk between litter and canopy, giving the species access to trunk-running ants and beetles that ground hunters and aerial web builders largely miss.

Seasonal behaviour: mature males abandon their tubes from about May to July in the southern states and wander in daylight, which is unusual for a mygalomorph and the reason almost every reported sighting is a male. A male that finds an occupied tube drums on the silk, is admitted, mates, and commonly dies inside within weeks. Females last several years and, on the evidence from captive atypids, possibly seven or more.

Relationship with humans: harmless and easy to live alongside. A tube on a garden tree damages nothing and removes insects for free, so leave it. Spraying is pointless, because the spider sits behind a sealed silk wall.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species name means red-footed, not simply red: Latin rufus for reddish plus pes for foot, a direct reference to the male legs.
• Purseweb spiders are among the very few spiders that bite through their own web. Most spiders let the silk hold the prey; an atypid stabs through the wall it is hiding behind, then repairs the hole.
• Pierre Andre Latreille described the species in 1829 in the catch-all genus Mygale; the parentheses in the modern authority record that later transfer into Sphodros.
• The genus itself spent most of the twentieth century sunk into Atypus and was revalidated only in a 1980 revision of the American Atypidae.

In summary, the red-legged purseweb spider is a harmless, secretive woodland mygalomorph that people meet only by accident. Learn the camouflaged tube at the foot of a trunk and the glossy black body on rust-red legs, and one of the most distinctive spiders in the southeast becomes unmistakable.