Kukulcania

Physical description and identification

Kukulcania is the American genus of the family Filistatidae, the crevice weavers. Around fifteen species are currently recognised, all of them confined to the New World, and they share one way of life: a silk-lined tube inside a hole, and a fan of dry woolly silk spread over the surface around it. The best known by far is the southern house spider (Kukulcania hibernalis); the rest are mostly regional spiders of the American Southwest, Mexico and South America.

Adults

• Size: females of the larger species reach about 19 mm (0.75 in) in the body; most are 10–15 mm. Males of the same species are 6–10 mm, far slimmer, and carry much longer legs.
• Color: females run from charcoal grey and brown-black to mid brown, with a velvety finish produced by a dense coat of short pale hairs. Males are lighter — tan, amber or reddish brown — and usually plain.
• Eyes: the family character. All eight eyes are crowded onto a single small raised tubercle at the front of the head, reading as one dark spot rather than the rows or pairs seen in most other families. Check this before anything else; it separates every Kukulcania from every recluse. See our guide to spider eyes.
• Distinctive features: mature males have long pedipalps that project forward past the head, tipped with swollen bulbs. Both sexes are cribellate, with a cribellum — a plate of thousands of tiny silk spigots — in front of the spinnerets and a calamistrum, a comb of bristles on each fourth leg, used to card that silk. The group is also haplogyne: females lack the hardened external genital plate of most araneomorphs, and species-level identification needs a microscope.

Eggs

Eggs are laid in a loose, roughly spherical sac of white silk kept deep inside the retreat rather than hung in the open web. The female stays with it, and a long-lived female produces several sacs across her adult life. In K. hibernalis a single sac holds in the region of 200 eggs.

Spiderlings

Spiderlings hatch pale and darken with each molt. They remain in the mother's web for a period and tolerate each other closely, sharing her silk and feeding together at a single prey item — a mild sub-social tendency that is genuinely unusual in spiders. Descriptions of organised group hunting go further than the evidence supports. Juveniles disperse on foot to their own crevices, often only a short distance away, which is why one wall can carry many retreats.

The web

Every species builds the same structure. A tube of silk lines a crack, hole or cavity, and from its mouth the spider lays down a radiating fan of lines across the surrounding surface, typically 10–20 cm across. The silk is cribellate: dry, woolly, glue-free, and it holds insects by snagging their bristles and claws in a mesh of nanofibres rather than by sticking to them. This is worth stating plainly, because the webs are often described as adhesive — they are not. An occupied site becomes a dusty grey sunburst over the years, because the spider adds silk rather than rebuilding. For the contrast with glue-based webs see our guide to spider webs.

Where Kukulcania spiders live

The genus is exclusively American. Its centre of diversity is the arid Southwest of the United States and northern Mexico, where most described species occur, including K. arizonica in Arizona and its neighbours, K. utahana, K. gertschi and the Baja California endemics K. bajacali and K. cochimi. Beyond that core the genus extends east across Texas to Florida, south through Central America and the Caribbean, and on into South America, with records reaching Chile, Peru, Venezuela, Brazil and Uruguay. Popular summaries of the genus that stop at Central America understate its southern extent.

Habitat is defined by the crevice rather than the vegetation, so the claim that these are tree spiders is misleading. They use cracks in rock faces and boulders, holes in adobe, brick, stone and stucco, gaps under window sills and behind shutters, the spaces beneath loose bark, and hollows in wood — anything that offers a dark cavity with a firm surface around the entrance. Human structures suit them so well that in much of their range they are more numerous on buildings than in natural habitat.

Are Kukulcania spiders venomous?

Yes, in the ordinary way: every species has venom glands and uses them on insect prey. None is medically significant to people. There is no antivenom for any Kukulcania, none is required, and no verified case of necrosis or systemic illness has been attributed to the genus. The venom is not related to that of the recluse spiders, which is the group people fear when they see the tan males, and it contains none of the tissue-damaging enzyme responsible for a genuine recluse lesion.

Can Kukulcania spiders bite?

They can, and bites are genuinely rare. A female's response to any disturbance is to reverse into her tube, and males run rather than stand. The recorded cases involve a spider crushed against skin — a hand pushed into a gap in stonework, or a wandering male caught inside clothing or bedding. The outcome is a brief pinch and a small red mark, occasionally with mild itching, which resolves in a day or two. Wash the site, apply a cold pack, and keep the spider or a photograph so the identification can be confirmed.

Ecological importance and behavior of Kukulcania spiders

These are sit-and-wait predators of exceptional patience. The spider spends the day inside its tube and moves to the entrance after dark, resting its front legs on the radiating lines. A beetle, moth, fly, ant or cockroach that crosses the woolly silk is located by vibration alone — the clustered eyes give poor resolution and play almost no part in hunting — and the spider rushes out, bites, and drags the prey back inside.

Natural predators: birds, lizards and geckos take exposed individuals, mud-dauber and other parasitoid wasps paralyse spiders to stock their nests, and centipedes, scorpions and larger spiders hunt them near their retreats. Wandering males are much the most vulnerable stage.

Prey-predator dynamics: because the webs sit on vertical surfaces where insects already concentrate — around lights, doorways, rock faces and eaves — a cluster of retreats works as free, permanent insect control, while the spiders themselves feed a range of desert and subtropical predators.

Longevity and seasonality: females are remarkably long-lived for araneomorph spiders, commonly persisting for several years and reportedly reaching about eight in the southern house spider, holding the same crevice throughout and continuing to molt and lay. Males mature, stop feeding, and leave to search, dying within weeks or months; this is why sightings of wandering spiders are so strongly seasonal while the webs themselves are permanent.

Relationship with humans: harmless and useful. If a silk fan is unwanted, brushing it off and sealing the crack behind it works far better than spraying, because the cavity is the resource the spider is holding.

Species in the genus and similar spiders

Quick facts

Did you know?

• The genus was erected in 1967 by the Finnish arachnologist Pekka T. Lehtinen, who split it from the Old World genus Filistata.
• The name honours Kukulkan, the feathered-serpent deity of the Maya — a male god, and not the serpent goddess of the retelling that circulates online.
• Filistatidae is one of the oldest surviving araneomorph lineages, and the cribellum its members carry is the ancestral spider silk-spinning organ, older than the sticky glue silk of orb webs.
• Female longevity in this genus is closer to a small tarantula's than to a typical house spider's, and a single crevice can be occupied continuously for most of a decade.

In summary, Kukulcania is a modest genus of about fifteen American spiders defined by one habit — living in a hole behind a fan of dry, combed silk. They are venomous only to insects, harmless to people, and their long-legged tan males are responsible for a large share of the brown recluse reports that turn out to be nothing of the kind.