Missing Sector Orb Weaver (Zygiella x-notata)

Physical description and identification

The missing sector orb weaver is a small, pale, very common spider that most people have lived beside for years without noticing. It is strongly tied to buildings: window frames, door surrounds, sheds, fences and bridge girders. The web usually identifies it first, because no other common European orb weaver leaves such a clean wedge of empty space in an otherwise perfect wheel.

Adults

• Size: females have a body length of about 5–7 mm and males about 3.5–5 mm. Even a large female is smaller than a fingernail, and the impression of size comes from the long front legs rather than the body.
• Color: the abdomen is silvery grey to cream, sometimes with a warm pinkish or yellow cast, and carries a dark, scalloped folium — a leaf- or oak-shaped central mark with wavy edges. The carapace is pale brown with a darker central band and dark margins, and the legs are yellow-brown with faint darker rings and dark spots at the joints.
• Eyes: eight eyes in the araneid pattern, four in a small central square and a pair set out to each side. Like most orb weavers it barely uses them; what it needs to know arrives as vibration along a thread.
• Distinctive features: the silvery sheen and the crisp dark folium separate it from the browner, bulkier relatives in Araneus. Males are slimmer, darker and longer-legged, with swollen pedipalps, and in autumn they are often found sitting quietly at the edge of a female's web.

Eggs and egg sac

After mating in late summer or autumn the female produces a loose, yellowish silk sac holding roughly one hundred eggs. She does not hang it in the open orb but hides it in the same crevice she uses as a retreat — a window rebate, a gap in mortar, under a windowsill — and covers it with a layer of tangled silk. She stays with the sac and defends it for as long as she lives, which in a sheltered spot can be well into the winter. The eggs themselves overwinter and hatch in spring.

Spiderlings

The young emerge in spring as pale spiderlings with only a hint of the adult folium. They scatter quickly, many of them by ballooning on a strand of silk lifted by warm air, which is how the species colonises new buildings so effectively. Juveniles build complete orbs with no missing sector at all; the gap appears only once they grow and start using a retreat.

The web

The web is a vertical orb 15–25 cm across, and in an adult one sector — almost always the upper one, nearest the retreat — is left free of the sticky spiral. Through that empty wedge runs a single signal thread from the hub straight to the spider's tubular silk shelter in a nearby crack. The spider sits inside with a front leg resting on the line; when an insect strikes the spiral, the tug travels down the thread and she rushes out along the clear gap without having to pick her way through her own sticky silk. Our guide to spider webs explains how the orb is laid out in the first place.

The sticky spiral is renewed frequently, often before dawn, while the frame threads and the retreat serve for weeks — which is why a web wrecked by rain is crisp again next morning in the same spot.

Where the missing sector orb weaver lives

Zygiella x-notata is native to Europe and the western Palaearctic, where it is one of the most abundant spiders on buildings from the Mediterranean north to Scandinavia. It is common everywhere in the United Kingdom, Ireland and continental Europe. Travelling easily with cargo and shipping, it is now established in North America — mainly the northeastern states, the Pacific Northwest and coastal Canada — and in southern Africa, South America, Australia and New Zealand.

The species is strongly synanthropic, meaning it thrives where people build things. Vertical surfaces with a crack alongside are ideal: window and door frames, brick pointing, fence posts, bridge parapets and boatyards. It also occurs on rock faces and tree trunks, but at much lower densities. In northern Europe it is one of the few orb weavers still found as an adult in midwinter, which is why it appears so often in lists of house spiders.

Are missing sector orb weaver spiders venomous?

Yes, in the ordinary sense: it has venom glands and uses them on the flies and midges it catches. The venom has no medical significance for humans or pets. Members of Araneidae do not possess the tissue-damaging enzymes of recluse spiders or the neurotoxins of widows, and this species is tiny, with fangs that struggle to penetrate human skin at all.

Can missing sector orb weaver spiders bite?

In practice, almost never. The spider is too small to bite most people effectively, and its response to a hand near the web is to bolt into its crevice or drop on a dragline. In the rare event of a bite — usually when a spider is squashed against skin — expect a faint pinprick and perhaps a small red spot that fades within hours. No systemic symptoms have been recorded.

Wash the area, apply a cold pack if it itches, and leave it. Medical attention is only warranted if the site becomes infected or if you show an allergic reaction, neither of which is caused by the venom itself.

Ecological importance and behavior of the missing sector orb weaver

This is a sit-and-wait predator on small flying insects, and its best hunting ground — a lit window on a summer night — is one humans created. Midges, mosquitoes, small moths, fungus gnats, aphids and flies make up almost all of the diet, and a single female on a kitchen window removes thousands of them over a season.

Natural predators: wasps are the main threat — spider-hunting wasps and mud daubers take adults from their retreats, and parasitoids lay eggs on juveniles. Birds pick spiders off walls, and larger spiders sharing the same wall eat smaller Zygiella.

Prey-predator dynamics: because it works vertical, artificial surfaces, it occupies a niche that vegetation-dwelling relatives such as Neoscona and the furrow orb weaver do not. On bridges and waterside buildings it often shares a wall with Larinioides species, which take the larger night-flying insects while Zygiella takes the small ones.

Seasonal behavior: the life cycle is annual. Spiderlings hatch in spring, mature from late July onward, and adults persist through autumn and — in sheltered positions — into winter. Males stop feeding, leave their own webs and take up residence at the edge of a female's web before mating. Females then produce one or more egg sacs and remain with them.

Relationship with humans: entirely benign. The webs are conspicuous on glass and are often cleaned away, but the spider rebuilds within a night or two. Leaving a few webs on outside frames costs nothing and cuts the number of midges and mosquitoes reaching the door.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species name x-notata means "marked with an X", a reference to the pattern on the abdomen rather than to the gap in the web.
• The empty sector is not camouflage and it does not hide the web from insects. It exists because the signal thread runs through it: a sticky spiral there would tangle the spider on every dash to the hub.
• Juveniles and spiders that build without a retreat make complete orbs. The missing sector appears only once the spider adopts a crevice to hide in.
• It is one of the very few orb weavers in northern Europe that can be found alive as an adult in December and January, sheltering in a window rebate.
• Carl Clerck described the species in 1757 in Svenska Spindlar, the work that also fixed the modern system of naming spiders.

In summary, the missing sector orb weaver is a small, harmless, remarkably successful neighbour. The gap in its web is not a flaw or a decoration but a piece of engineering — an open corridor between a hidden spider and the trap it is listening to.