Marbled Cellar Spider (Holocnemus pluchei)

Physical description and identification

The marbled cellar spider is a pholcid — one of the true daddy-long-legs spiders — but it is stockier and more strongly patterned than the pale, elongated species most people picture. It hangs upside down beneath a curved sheet of silk, and it is very often not alone.

Adults

• Size: both sexes measure roughly 4–7 mm in body length, with females averaging slightly larger and heavier. The leg span reaches about 5 cm, so the spider looks far bigger than it is.
• Colour: grey to pale tan, mottled and marbled with cream and darker brown on the upper surface of the abdomen. The decisive feature is underneath: a broad dark brown or blackish stripe running lengthwise along the belly of the abdomen. No other common cellar spider in its introduced range has it.
• Eyes: eight, in the standard pholcid arrangement of two raised groups of three with a smaller pair in front.
• Distinctive features: the abdomen is oval and relatively short rather than the long cylinder of Pholcus phalangioides. The legs are pale with darker rings at the joints, giving the banded look people notice first. Both sexes carry stridulatory files and can rasp audibly, and mature males have swollen palps and projecting jaws.

Eggs and egg sacs

As in all pholcids, the female wraps her eggs in a bare minimum of silk and carries the bundle in her jaws until it hatches, roughly three weeks later. Clutches typically hold a few dozen eggs rather than the two hundred sometimes quoted — that higher figure is closer to a female's whole lifetime output across several successive clutches. She cannot feed while carrying a bundle, so she stays in the web and relies on stored reserves.

Spiderlings

Hatchlings are pale and translucent with short legs. They remain in the web for a short period, then either build a small sheet of their own nearby or move into an existing web occupied by others — the behaviour that makes this species so conspicuous where it is established. They mature through a series of moults, with the legs lengthening faster than the body each time.

The web

The web is a shallow, curved sheet of non-sticky silk slung inside a loose tangle of scaffolding threads, with the spider hanging inverted below the sheet. Females carrying eggs favour the deeper, more domed parts of the structure. Prey is not caught by glue: an insect strikes the scaffolding, falls onto or against the sheet, and the spider flicks silk over it from a distance with its hind legs before biting. Webs are added to rather than rebuilt, and individuals often abandon one web for another several times in their lives. See our guide to spider webs.

Where the marbled cellar spider lives

Holocnemus pluchei is native to the Mediterranean basin — Italy, Spain, France, Greece, the Balkans, Turkey, the Levant and North Africa — where it lives in caves, rock crevices, dry stone walls, ruins and outbuildings. It reached California in the second half of the twentieth century and spread rapidly; it is now one of the most abundant spiders around buildings in much of the state and is established through Arizona, Nevada, Utah and parts of Texas, with records elsewhere in the Americas.

Its preferred sites are sheltered but not necessarily indoors: the underside of eaves, porch ceilings, window recesses, garden walls, fence corners, woodpiles, sheds, garages, water meters, culverts and the gaps behind stacked pots. It tolerates hot, dry summers better than most pholcids and is more of an outdoor spider than the long-bodied cellar spider, which is why the two can occupy the same property without competing directly. Where it does come indoors it behaves like any of the common house spiders, preferring dim, still corners; our guide to where spiders live covers the habitat preferences of the group.

Are marbled cellar spiders venomous?

Yes, in the same sense as almost every spider: Holocnemus pluchei has venom glands and uses venom to kill the insects it catches. It is not dangerous to people, and the venom is weak even by spider standards, with none of the neurotoxic or tissue-damaging components that make widow and recluse bites medically important.

Because this species is also called a daddy-long-legs, it attracts the same tired myth as its relatives: that it is the most venomous spider in the world but has fangs too short to break skin. Both halves are false. No study has ever shown pholcid venom to be unusually potent to mammals, and the fangs — although short, at about a quarter of a millimetre — are comparable to a brown recluse's and can in fact penetrate human skin. A televised test in 2004 confirmed a genuine bite from a cellar spider, and the result was a faint burning sensation lasting a few seconds. Weak venom, trivial bite: that is the whole of it.

Can marbled cellar spiders bite?

In principle yes, in practice almost never. The spider is small, fragile and deeply committed to its web; disturbed, it whirls until its body is a blur or drops out of the web on a silk line. Getting bitten requires trapping one against skin, which is difficult to do by accident.

Any bite that did occur would amount to a momentary pinprick with perhaps a small red mark that fades within the hour. There are no reports of necrosis, systemic symptoms or allergic emergencies from this species, and no antivenom exists because none is needed. Wash the area and use a cold pack if it is sore; seek advice only if redness spreads over subsequent days, which would indicate an ordinary skin infection rather than venom. Cats and dogs that eat these spiders are unaffected.

Ecological importance and behavior of the marbled cellar spider

Living in groups: the most interesting thing about this species is its tolerance of company. Individuals frequently settle in webs already occupied by others of their kind, so a porch ceiling can carry a loose colony of overlapping sheets. Grouped spiders gain a larger combined capture area and better early warning of predators; they pay for it in competition over prey and in occasional cannibalism. Group members also move house often, abandoning one web for another when catches fall — behaviour that has made the species a favourite subject for research on the costs and benefits of group living in spiders.

Stridulation: both males and females have stridulatory organs and rasp them during courtship and aggressive encounters. Sound production by both sexes is unusual; in most spiders that stridulate at all, only the male does.

Diet and defence: the web takes flying and crawling insects — houseflies, fungus gnats, mosquitoes, moths, ants, small beetles — and other spiders, including smaller pholcids. Threatened, the spider gyrates rapidly in the web, blurring its outline, and only bolts or drops if that fails.

Natural predators: jumping spiders are the classic enemy, since they can walk the web and stalk the occupant. Larger pholcids, including Pholcus phalangioides, invade the webs and take residents; parasitoid wasps, geckos, lizards, birds and ants take their share too.

Seasonal behaviour and humans: in mild climates the species is active year-round with a summer breeding peak, and individuals live one to three years. Its relationship with people is straightforwardly beneficial — free control of flies and mosquitoes around a building. Where webs are unwanted, sweeping or vacuuming them is enough; spraying achieves little, since new spiders move in within weeks.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species was described in 1763 by Giovanni Antonio Scopoli, the naturalist and physician who also named a great deal of Europe's insect fauna, making it one of the oldest named spiders in the family.
• Both sexes stridulate. In the great majority of spiders that make sound at all, only males have the apparatus.
• A single porch ceiling in California can carry dozens of overlapping webs — a density that its solitary relatives never reach.
• Group members regularly move house: an individual that catches little will abandon a shared web and try another, sometimes several times in a season.
• The dark belly stripe is the fastest field check there is. Look at the spider from below and the identification takes a second.

In summary, the marbled cellar spider is a small, harmless, sociable Mediterranean pholcid that has made itself at home across the American south-west. It is worth knowing for its belly stripe, its shared webs and its rasping courtship — and not at all worth worrying about.