Pholcus

Physical description and identification

Pholcus is the type genus of the cellar spider family Pholcidae and by far its largest group. The World Spider Catalog listed 321 species in 2016; the figure has passed 350 since, and new species are described from Asian caves and African forests every year. Recognising the genus is easy; recognising a species usually is not.

Adults

• Size: body length runs from roughly 2 mm in small forest species to about 10 mm in the largest, with most between 5 and 9 mm. Leg span is several times the body length and reaches 7 cm in the familiar house species. Males are slightly smaller and slimmer than females.
• Colour: pale and low-contrast — cream, straw, grey or light brown, usually semi-transparent, often with a darker central mark on the carapace and faint grey blotches along the abdomen. Cave-dwelling species tend to be the palest of all.
• Eyes: eight in the standard pholcid layout, two raised groups of three with a small pair in front. A few cave species have reduced eyes, and at least one, Pholcus caecus, is effectively blind.
• Distinctive features: an elongated, often cylindrical abdomen, a rounded carapace and extremely thin legs ending in a flexible, finely divided tip that grips a single thread. Females are haplogyne — they have no hardened external genital plate — so species identification rests on the shape of the male palp and the internal female structures under magnification.

Eggs and egg sacs

Across the genus the female binds her eggs into a loose bundle with only a few strands of silk and carries it in her jaws, not her pedipalps, until the spiderlings emerge. Clutch size varies with species and female size, commonly between about 15 and 60 eggs, and a female may produce two or three clutches in a season. Incubation takes roughly two to four weeks.

Spiderlings

Hatchlings are transparent, with legs short in proportion to the body. They stay clustered around their mother for about a week, then drift apart, and sibling cannibalism is common where they do not disperse fast enough. Five or six moults follow before maturity, with the legs lengthening disproportionately at each one. Our guide to the spider life cycle covers the process in more detail.

The web

Every Pholcus builds a loose, irregular, three-dimensional tangle and hangs upside down beneath it. Descriptions of the web as flat or sheet-like are misleading: the silk is a scaffold in space, not a plane. None of it is sticky. Prey is caught when the threads collapse around it and then wrapped from a distance with silk flicked by the hind legs before the spider risks a bite. Webs are never dismantled or eaten, so they accumulate layers and dust. See our guide to spider webs.

Where Pholcus spiders live

The genus occurs on every continent except Antarctica, but that global picture is misleading. Most Pholcus species have small ranges — a single mountain, cave system or patch of forest — and the centre of diversity is tropical and subtropical Asia, with a second concentration in Africa and around the Mediterranean. China, Korea, Japan, Laos, Thailand, Indonesia, Malaysia and the Philippines between them hold a large share of the described species, and so do Cameroon, Tanzania, the Canary Islands and Iran.

Only a handful of species live with people, and they are the reason the genus looks cosmopolitan. Pholcus phalangioides has reached almost every country through trade; P. manueli is established in North America and East Asia, and P. opilionoides is widespread in Europe. These are the species found in cellars, garages, sheds, barns, stairwells and ceiling corners, and they make the genus a fixture among house spiders.

Wild species favour shaded, humid, sheltered spaces: caves and rock overhangs, hollow logs, the undersides of boulders, leaf litter and the space beneath curled dead leaves in forest understorey. Humidity matters more than warmth, and freezing temperatures exclude the genus from cold regions except inside heated buildings.

Are Pholcus spiders venomous? The daddy-long-legs myth

Yes, in the ordinary sense: Pholcus spiders have venom glands and use venom to subdue insects and other spiders. They are not, however, the most venomous spiders in the world, and the story that says otherwise is the most persistent piece of misinformation in arachnology.

The myth has two parts and both are wrong. No study has ever shown pholcid venom to be extraordinarily potent to mammals; it is an insect venom with none of the neurotoxic or tissue-destroying components that make widow and recluse venoms medically important. And the supposed safeguard — fangs too short to pierce skin — does not hold either. Pholcid fangs are short, about a quarter of a millimetre, but so are those of the brown recluse, which bites people regularly. The arachnologist Rick Vetter argued this for years, and in 2004 MythBusters tested it on camera: the spider bit a human arm and produced a mild burning sensation that lasted a few seconds. Weak venom and a trivial bite is the whole story; no member of the genus belongs on a list of the most venomous spiders.

Can Pholcus spiders bite?

They can, and in practice almost never do. These are fragile, slow-moving spiders that respond to a threat by whirling in the web or dropping on a silk line; a bite requires the spider to be pressed against skin with nowhere to go. The few recorded bites describe a brief pinprick or faint burning that fades within minutes, sometimes with a small patch of redness. There are no documented cases of necrosis, systemic illness or allergic emergency from any Pholcus species, and no antivenom exists because none is needed.

Treatment is simply soap and water and a cold pack if the spot is sore. Seek medical advice only if redness spreads over the following days, which indicates a skin infection rather than venom. Pets that catch and eat these spiders are unharmed.

Ecological importance and behavior of Pholcus spiders

The genus is best known to biologists for araneophagy — eating other spiders. A Pholcus enters a neighbouring web and plucks the silk in a rhythm that imitates a trapped insect; when the resident comes to investigate, the intruder throws silk and bites from beyond the reach of its shorter legs. This is textbook aggressive mimicry, and it lets a spider weighing a fraction of its victim kill house spiders, cobweb spiders and funnel weavers. The ordinary diet is broad: mosquitoes, midges, flies, moths, woodlice, silverfish, ants and small beetles.

Defensive whirling: disturbed, the spider braces its legs and gyrates until the body is an indistinct blur, then stops dead. The behaviour is shared across the family and gives the group its other common name, vibrating spiders.

Natural predators: other pholcids — including larger individuals of the same species — are the most consistent threat. Jumping spiders hunt them inside their webs, and parasitoid wasps, centipedes, geckos, ants and birds take them where ranges overlap. Spiderlings are the most vulnerable stage.

Seasonal behaviour: indoor species breed year-round with a mild late-summer peak; wild tropical species are active whenever humidity allows. Longevity is unusual for spiders of this size, with two years normal and three possible.

Relationship with humans: entirely benign. The synanthropic species remove biting flies and mosquitoes from buildings at no cost, and clearing their webs with a vacuum is more effective than any insecticide, since new spiders simply walk back in.

Species in the genus and similar spiders

Quick facts

Did you know?

• The genus was erected in 1805 by Charles Athanase Walckenaer, a French naturalist and administrator who named a large share of Europe's spider fauna.
• Pholcus caecus, described from a cave, carries a name meaning blind — several cave species have reduced or vestigial eyes.
• The aggressive mimicry used by these spiders is one of the classic laboratory demonstrations of deceptive signalling in invertebrates.
• Most species are narrow endemics, so a genus famous for a global house spider is mostly made up of animals found in one valley or cave.
• The extremely long first legs are not for running. They are reach: the spider uses them to touch, silk-wrap and bite prey from a distance that keeps its own fragile body safe.

In summary, Pholcus is a huge, mostly tropical genus that the world knows through a single cosmopolitan species. They are weak-venomed, effectively harmless, remarkably long-lived, and far more dangerous to the other spiders in a building than to anyone living in it.