St Andrew’s Cross Spider

Some Facts about the St Andrew’s Cross Spider

The female St Andrew’s Cross Spider bite can pack a mighty and deadly punch for her male mating partner.

This is driven by a need for survival because scientists have proven that male spiders in her web cause her to attract less prey and attract more predators! Yikes….

standrewsspider2

  • Scientific name – Argiope keyserlingi
  • This is known as an ORB Spider
  • Lifespan – 1 year
  • Diet – flying insects
  • Toxicity – none, harmless to humans
  • Habitat – suburban gardens and bushland (likes lomandra bushes)
  • Body length approximately 5 mm for male and 12 to 15 mm for female
  • Light to dark brown in color.
  • This spider constructs a large web (a few meters off the ground) usually found in summer in garden areas around the home.
  • Web: An orb web with up to 4 zigzags of silk forming a cross (hence the name) that radiates out from the spiders central position
  • Identification points: abdomen striped yellow and brown and forms a cross in the middle of the web.

It takes the female about an hour to build her web each day because she will have eaten the previous one for its protein.

But the burning question is….

What is the Cross for?

The 3 most likely functions are:-

  1. it attracts prey because the decorations reflect ultraviolet UV light very strongly (whereas the surroundings don’t) and the so there is a very strong contrast between the web and the garden or bush background
  2. it deters predators (e.g. Friar Birds)
  3. and it deters non-predators that otherwise may have destroyed the web by accident (e.g. Humans)

There is a Conflict, hence webs will vary. The Praying mantid is a predator and is also attracted to the UV light of the web design because it has a similar visual system to that of the honeybees and flies which are common prey for our spider. The clever St Andrew’s Cross spider will vary the number of bands it uses on a daily basis.

So in summary: The role of the spiders web in the cross-shaped pattern, which is called a stabilimentum, has challenged the spider experts. It was a theory for a long time that it strengthened the web, however it is now thought that it helps to attract prey. But how exactly; well because the threads of the cross reflect ultraviolet light in a similar fashion to some plants. It’s believed that flying insects mistake this ultraviolet light for a flower and are attracted onto the web.

Breeding

The eggs of the St Andrews Cross spider are laid in a silk egg sack, which is normally put in leaves or twigs near the mother’s web. When the little spiders hatch, they make their webs with a white disk in the middle, then add the cross as they get older. When they are fully grown, they only make the cross in the web.

I am seeing these webs everywhere at the moment as we walk in the bush each morning!

Kerrie