Araneus gemma

Physical description and identification

Araneus gemma is a medium-large western North American member of the genus Araneus, and one of the group that people call cat-faced spiders. The nickname comes from the shape of the abdomen seen from the front: two blunt humps stand up like ears, and the dimpled muscle attachment points below them read as a pair of eyes and a nose. It is an accident of anatomy, but once seen it is hard to unsee.

Adults

• Size: mature females have a body roughly 10–16 mm long, swelling considerably as the eggs develop, with a leg span two to three times that. Males are smaller and much slimmer, about 6–8 mm, with longer legs and clubbed pedipalps.
• Colour: grey, fawn, tan or a dull brownish purple, usually with a pale median stripe of variable length down the abdomen and a scalloped darker folium along the sides. The colour is a camouflage pattern rather than an identifying feature, and individuals from the same web frame can look quite different.
• Eyes: eight eyes in two rows, the four middle ones set in a small square on a slight mound. As in all orb weavers, the eyes are small and the spider depends on touch and web vibration rather than sight.
• Distinctive features: the paired anterior humps are the mark to look for, together with a bulky, high-shouldered abdomen and short, sturdy, faintly banded legs. Confident separation from the closely similar cat-faced spider (Araneus gemmoides) is a job for a microscope and adult genitalia; in the field, location helps more than appearance, as A. gemma is the coastal and far-western form.

Eggs and egg sac

Females mate in autumn and lay a single large clutch before the weather turns. The eggs go into a rounded sac of orange to buff silk, tucked under an eave, a windowsill, a fence rail or a flap of bark close to where the female has spent the season. She may stay beside it for a while, but she does not survive the winter.

Spiderlings

The eggs overwinter inside the sac and the spiderlings emerge in spring, brown with bright yellow abdomens that look nothing like the muted adults. They hang in a dense cluster for a day or two, moult, then climb to exposed points, spin out silk and drift away on air currents — the process described in our guide to spider ballooning. They grow through spring and summer and only become noticeable in late summer, when the survivors are large.

The web

The web is a vertical orb, typically 30–50 cm across, slung across a doorway, a porch corner, a gap between fence posts, or a space in coastal scrub. The species is mainly nocturnal: the spider builds or repairs the orb at dusk, waits at the hub through the night, and near dawn takes much of the silk down and retires to a rolled leaf, a crevice or the shelter of a window frame at the end of a signal line. Dragline silk in this genus is roughly 1–4 micrometres thick — a few hundredths of the width of a human hair — and still strong enough to stop a flying beetle. See our guide to spider webs for how the structure is assembled.

Where Araneus gemma lives

Araneus gemma is a spider of western North America. It is common along the Pacific slope from southern British Columbia through Washington and Oregon into California, and extends inland into the drier interior of the Pacific Northwest and the northern Great Basin. It does not occur in eastern North America, where other Araneus species fill the same role.

Habitat is broad but always structured: coastal dunes and scrub, open woodland and forest margins, chaparral, riverbanks, hedgerows, gardens and the outsides of buildings. Porches, carports, garden sheds and window frames are prime sites because they combine firm anchor points with lights that pull in moths. The species is most conspicuous in September and October, which is why it is one of the spiders people most often ask about at the start of autumn on the west coast.

Is Araneus gemma venomous?

Yes, in the ordinary sense that it uses venom on its prey, and no in the sense that matters: the venom has no medically significant effect on humans, dogs or cats. Orb weavers of the genus Araneus rely on toxins tuned to insect nervous systems, they do not have tissue-destroying enzymes like the recluse spiders, and there is no antivenom for any of them because none has ever been needed.

Can Araneus gemma bite?

It can, but almost never does. The first response of a disturbed orb weaver is to shake the web, then to drop off it on a silk line and lie still on the ground. Bites are recorded only when a spider is picked up, pressed inside gardening gloves, or trapped against skin while someone is clearing a web by hand.

A bite produces brief local pain, a small red mark and perhaps slight swelling, comparable with a nettle sting and gone the same day. Wash the site with soap and water and use a cold pack if it is sore. Medical attention is appropriate only if the wound looks infected after a day or two, or if there are signs of an allergic reaction such as widespread hives, dizziness or difficulty breathing.

Ecological importance and behavior of Araneus gemma

Araneus gemma is a sit-and-wait predator whose entire strategy is the web. The spider holds a signal thread from its daytime retreat, or sits head-down at the hub after dark, and reads the tension in the radii. Prey is bitten first and then wrapped, and small items are eaten at once while larger ones are stored at the hub.

Diet: flying insects, with a strong bias toward whatever the site supplies — moths, crane flies, midges, mosquitoes, flying ants, small beetles, leafhoppers and wasps. A web beside a porch light is a highly productive trap, and a female in autumn eats almost continuously to convert insects into eggs.

Natural predators: birds, particularly those hunting at dawn and dusk when the spider is still exposed; mud-dauber wasps that stock their nest cells with paralysed orb weavers; parasitoid wasps that lay on the spider's abdomen; and shrews, lizards and larger spiders. Egg sacs are raided by parasitic wasps and by birds picking under eaves.

Mating and seasonal behaviour: males mature in late summer, abandon their own webs and go looking. A male approaches on a mating thread at the edge of the female's orb, signals by plucking, and mates quickly; he does not usually survive long afterwards. The species is annual: adults die in late autumn and the population passes the winter entirely as eggs in silk sacs.

Relationship with humans: harmless and useful. If a web is in an inconvenient place, taking down the frame lines in the evening encourages the spider to relocate; there is no reason to use insecticide on a spider that is removing mosquitoes for free.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species was described in 1888 by Henry Christopher McCook, an American clergyman and naturalist whose three-volume study of American spiders was among the first serious accounts of web-building behaviour anywhere.
• The name gemma is Latin for a bud or a jewel, which is how the swollen, faintly iridescent abdomen of a well-fed female looks in low sun.
• The "cat face" is made of muscle attachment points. The dimples that read as eyes are the places where internal muscles anchor to the abdominal wall, and they appear on every member of this group.
• Dragline silk of this thickness — one to four micrometres — is far finer than a human hair, yet weight for weight it absorbs more energy before breaking than steel does.
• Autumn spiders on porches are not new arrivals. They hatched in spring, spent the summer as small spiders nobody noticed, and simply became large enough to see at the same time as their webs became large enough to walk into.

In summary, Araneus gemma is a harmless, hard-working western orb weaver that trades a night's silk for a night's mosquitoes. The humped abdomen names the group, the location narrows it down, and nothing about it warrants a spray can.