Monday, 5 November 2007

#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.


Esperance Blog said...

Where did the saying 'It's a man's world' come from? Not from the St Andrews mob, that's for sure. Very interesting Gaye, thanks.


Gaye from the Hunter said...

One thing's for sure, that saying did not come from the world of spiders :)

As I'm researching invertebrates, it will be interesting to note if any others (besides praying mantids) practice sexual cannibalism.


David said...

Great photos and commentary Gaye -- not my favourite subject, but I'm a wimp when it comes to our eight-legged friends.

Gaye from the Hunter said...

Thank you David. Spiders do take a bit of getting used to, but unless they are inside my home, amongst my clothing, shoes or hair, I am very tolerant of them now that I have observed them at length to appreciate the complexity of their lives.

I was bitten by a spider for the first time a couple of days ago. I was mowing, and a small brown spider found its way into my shirt as I brushed passed a row of bottlebrush shrubs. I wasn't aware of the spider until I felt a pain like that of a very sharp pin prick. My skin turned red and itchy over an area of about the size of a 50cent piece immediately. But half an hour later, the redness and itchiness had almost disappeared, and I had no other symptoms. I was unable to identify the spider.


David said...

Yet again Gaye your photos with their crystal focus, well arranged subject matter and depth of colour continue to amaze and astound.
Thank you, you help me to visualise what is possible.

David said...

Hi again Gaye,
Just a thought on the stabilimentum...When a Snt Andrews Cross is disturbed, it will arch itself away from the centre of the web....only making contact with four paired legs at the edge of the "X".
I would imagine that any bird that tried to access the spider through that "wrap" silk would get in quite a mess.
Along with the other reasons mentioned in the linked article, discouragement through discomfort may be part of the structures purpose also.



Gaye from the Hunter said...

hello David, and thank you.

I will admit that I have left my SLR digital idle for some time while using my little compact digital. Although my compact camera does have a macro function, I have been unable to get any worthwhile results from it, and always just use normal settings and move in close. I am extremely happy with my little point-and-shoot camera. Focus can be very tricky, but I have experimented to the point that I am reasonably confident. Over recent months I have also learned how to fire the flash even when the camera doesn't think it needs it, so this has been a great help to me.

The variation in the stabilimentum is quite amazing. I haven't had a St Andrew's Cross Spider hang around my yard long enough to make detailed observations regarding defense strategies - but I always hope.

Thank you for your comment, and seeasons greetings to you and yours - stay safe.


Anonymous said...

Where I live, we often get many Saint Andrews Cross Spiders throughout our backyard. At one stage we had between 50 and 75 in a relatively small area.