#36 A colourful bush and urban spider

Although the St Andrew's Cross spider is apparently a common species, I have not often encountered it, so when a female chose to occupy a shrub in my backyard, I was extremely pleased to have the opportunity to observe some aspects of its life.

Unfortunately, the spider disappeared shortly after, following a wild storm with very heavy rain. But she left two egg sacs intact.

Argiope keyserlingi - St Andrew's Cross Spider

Argiope keyserlingi, St Andrew's Cross Spider, is found in the east of Australia in Queensland, NSW and Victoria in a variety of habitats. A mature female has an ornate yellow and red, reddish-brown or black striped abdomen, with irregular yellow markings and spots on the underside.

The palps (pedipalps) of female St Andrews Cross Spider

An adult female can have a body length of up to 20mm, while plain brown males have a body length of 4 or 5mm.

Web of St Andrew's Cross Spider

According to Christian history, the Apostle St Andrew, was crucified on an X-shaped cross rather than the traditional crucifixion cross. The St Andrew's Cross Spider was named after this reference in relation to the X-shape formed by the spider's outstretched legs at rest.

An adult female will build a vertical orb web up to about 1 metre in diameter in low shrubs or other low vegetation. She will decorate the web with white silk 'zigzags' forming a 'X' or part thereof. This decoration is sometimes known as a stabilimentum. A young female may, instead, construct haphazard circular stabilimentum. Dispite the sustained attention of researches, the exact function of these decorations is yet to be understood.

Green silk egg sac of St Andrew's Cross Spider

The St Andrew's Cross Spider is active both day and night, occupying the centre of her web. One or more males will wait on the perimeter of the web strumming his mating intentions on the web.

Females construct more than one greenish silk cocoon hidden amongst the foliage close to the web. Hundreds of eggs are deposited in the egg sacs in autumn or late summer.

Argiope keyserlingi (St Andrew's Cross) spiderlings

The St Andrew's Cross Spiderlings emerged from egg sacs in a small Leptospermum shrub in my backyard in July, dispersing on the breeze early in August. Spiderlings are pale grey in colour. They exit the egg sac by unsecuring the flat back of the 'teardrop'-shaped structure.

Sexual cannibalism

Sexual cannibalism is common amongst spiders, but studies have determined that sexual cannibalism differs in the world of the St Andrew's Cross Spider. Females do not consume the captured male while copulating, unlike other sexually cannibalistic spiders, but wrap him in silk, thus terminating copulation.

Females apparently delay cannibalising their second mate if he is relatively smaller than the first, which results in the second male fertilising a larger proportion of her clutch of eggs. Therefore, it has been shown that females adjust the paternity of a preferred male through the timing of cannibalisation.

The web of St Andrew's Cross Spider

It has also been shown that males do not comply to sexual cannibalisation, as is the case with some other spider species. Males that have survived a mating encounter are often missing legs, indicating that they have struggled to avoid capture.

Studies have also revealed that St Andrew's Cross Spiders will relocate their web if the web is damaged by large non-prey animals or if limited prey is available.

A brilliantly coloured St Andrew's Cross Spider

These showy spiders lead complex and interesting lives. I have only encountered two adult female St Andrew's Cross Spiders in the Hunter Valley, indicating that they are not as common as many other garden spiders. If you are lucky enough to have one decorating a corner of your backyard, enjoy its brief stay.

My spider index links to all other Hunter Valley spider species that I've featured on my nature blog. And here is a useful site for identifying common spider species.