St. Andrews Cross (Argiope keyserlingi)

Physical description and identification

The St Andrews cross spider is a mid-sized member of the Argiope garden spiders, a worldwide group of boldly banded orb weavers. In eastern Australia it is by far the most familiar of them. Once you know the posture — head-down at the centre of the web, legs pressed together in four pairs pointing to the corners — the species is unmistakable, because it spends nearly all day in exactly that position.

Adults

• Size: females measure 10–16 mm in body length, males only 3–4 mm. The leg span of a mature female is roughly the width of a matchbox; the male is easy to overlook at the edge of her web.
• Colour: the female's carapace is silvery-grey with fine white hairs. The upper abdomen carries bold transverse bands of yellow, silver-white and black, often with a reddish-brown or maroon band toward the front. The underside is dark brown to black with two long yellow stripes running lengthways. Males are plain cream to light brown with faint darker lines and no bright banding.
• Legs: banded dark brown and pale, and noticeably long. At rest they are joined in pairs, so a resting female appears to have four legs, not eight.
• Eyes: eight small eyes in two rows, none enlarged. Like other orb weavers it hunts entirely by vibration rather than sight.

Eggs and egg sac

After mating the female produces a pear-shaped or flask-shaped sac of green-brown silk containing roughly 400–1,000 eggs. She does not leave it in the capture spiral: the sac is suspended among leaves and stems close to the web, where its colour blends with the foliage. The eggs hatch during autumn, but the spiderlings do not leave — they overwinter inside the sac and emerge the following spring.

Spiderlings

Emerging spiderlings are pale with a faint dark pattern and stay clustered for a few days before dispersing on silk threads. They build small orbs almost immediately, and even at a few millimetres across many add a silk decoration, though juveniles frequently spin it as a spiral or a disc rather than a cross. Growth proceeds through a series of molts, with females reaching adult size by mid to late summer.

The web

The web is a vertical orb, usually 30–40 cm across, built in shrubs, garden beds, between fence rails and across the gaps in low vegetation, normally less than two metres from the ground. The centre carries a stabilimentum: bands of dense, bright white silk laid in four rays that form the diagonal cross. A full web takes the better part of a day to build, and the spider usually replaces the capture spiral every day or two while keeping the frame.

What the cross is for has been studied for decades and there is no single answer. The dense silk reflects ultraviolet strongly, and experiments show that decorated webs intercept more flying insects than undecorated ones, so prey attraction is real. The same conspicuousness also warns birds away from flying through the web, and the bands may break up the spider's outline against a predator's view. Individual spiders change how much decoration they add depending on hunger, light and recent disturbance, which fits a structure serving more than one purpose. For the wider picture, see our guide to spider webs.

Where the St Andrews cross spider lives

Argiope keyserlingi is an eastern Australian species. It is found along the coast and ranges from northern Queensland south through the New South Wales coast to roughly the Sydney–Illawarra region, with scattered records further south. It is one of the commonest large orb weavers in suburban gardens in Sydney and Brisbane, and it has also been recorded on some Pacific islands east of Australia.

It favours warm, sheltered, sunlit gaps in low vegetation: open eucalypt forest, rainforest margins, heath and shrubby regrowth, and — very readily — hedges, garden beds, citrus trees and verandah corners. Webs are built low, where flying insects funnel between plants. Numbers peak in summer and early autumn, and adults largely disappear over winter, which is why the spider seems to arrive suddenly each year.

Are St Andrews cross spiders venomous?

Yes, in the ordinary sense: the spider has venom glands and uses them on insects. It is not dangerous to people. No serious envenomation has been recorded from Argiope anywhere in the world, there is no antivenom because none is needed, and the species is not related to the funnel-webs or redbacks that account for Australia's medically significant bites.

Can St Andrews cross spiders bite?

They can, but almost never do. A female's escape response is to drop straight out of the web on a dragline, or to shake the whole orb rapidly so that her outline blurs. Bites happen only when a spider is squeezed — for instance when someone gardens bare-handed and closes a hand around a leaf she is sitting on.

The result is a brief local sting with slight redness, resolving within hours and without blistering or ulceration. Wash the site, use a cold pack if it is sore, and keep an eye on it. Medical attention is worth seeking only if the pain escalates, redness spreads over the following days, or there are signs of a general allergic reaction. If you were bitten in the garden and are unsure what bit you, photograph the spider rather than trying to catch it.

Ecological importance and behavior of the St Andrews cross spider

This is a sit-and-wait predator that stays in the web day and night. Prey — flies, moths, butterflies, bees, small beetles, grasshoppers and the occasional dragonfly — is detected by vibration, rushed, wrapped in a broad sheet of silk and then bitten. A single female in a productive garden takes hundreds of insects across a season, which makes her a useful, free pest control service.

Defence: besides dropping from the web, disturbed adults perform a fast vibrating motion that makes them almost impossible to focus on. The bright banding on the upper surface may also advertise that the spider is unpalatable or simply not worth chasing.

Mating: mature males build no webs of their own and instead take up residence at the edge of a female's orb, often several at a time, plucking the silk in a courtship rhythm before approaching. Mating is dangerous: the male's palp frequently breaks off inside the female and is left behind as a plug, and males are often cannibalised during or just after their second copulation. The genus is a long-standing model system for research on sexual cannibalism.

Predators: birds, praying mantises, spider-hunting and mud-dauber wasps, and larger spiders. Parasitoid wasps attack both adults and egg sacs.

Season and lifespan: spiderlings emerge in spring, females mature over summer, egg sacs are produced in late summer and autumn, and adults die before winter. The whole cycle runs about a year, with males living only a few weeks after their final molt.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The name honours St Andrew, who by tradition was crucified on a diagonal cross — the same saltire that appears on the flag of Scotland.
• The species name commemorates Eugen von Keyserling, the nineteenth-century German arachnologist who described many Australian spiders.
• The four-armed decoration is not fixed. A hungry spider adds more silk to it, and a spider that has just been disturbed by a bird may leave it out altogether.
• Because the male's palp often snaps off and plugs the female, many males effectively mate only once — and a female may carry the broken palp of her first suitor for the rest of her life.
• Web decorations of this kind evolved independently in several unrelated orb-weaving groups, which is strong evidence that they do something useful.

In summary, the St Andrews cross spider is a harmless, hard-working garden predator with one of the most recognisable webs in Australia. If a bright banded spider is sitting in an X at the centre of a low orb web, you have found one — and the best thing to do is leave the web where it is.