8 Things You Should Know About the St. Andrew's Cross Spider

The St. Andrew's cross spider is a large orb-weaver spider found across much of eastern Australia. It belongs to the genus Argiope, whose members are famous not just for their size, but also their often brightly colored abdomens and the distinctive zig-zags they weave into their webs.

Here are a few things you may not know about this eye-catching arachnid.

1. They Are Named After a Crucifixion

The zig-zags in the web of a St. Andrew's cross spider form a large X shape, similar to the heraldic symbol known as a saltire. It's also known as Saint Andrew's cross, since Andrew the Apostle is traditionally said to have been crucified on a diagonal cross in the shape of the letter X. When the spider sits in the middle of the cross, it can look as though it's suffering a similar fate. (In reality, of course, that fate is reserved for the spider's prey.)

2. The Cross Might Help Them Catch Prey

The web decorations of Argiope spiders have long posed a mystery, and there is still no clear consensus about their purpose. They're called stabilimenta, a reference to an early belief that they help fortify or stabilize the web. More recent research suggests these elaborate designs have less to do with the web's structure, however, than its appearance.

The cross of a St. Andrew's cross spider is woven with a bluish-white silk that strongly reflects ultraviolet light. Many flying insects are attracted to UV light, which can help them find flowers or fly through dense foliage, so the cross might lure unwitting prey into the spider's clutches. On the other hand, some research suggests stabilimenta might actually reduce prey capture, suggesting these web decorations serves another purpose.

3. The Cross Might Intimidate Predators, Too

Another theory suggests the cross helps protect the spider from predators, which might sound counterintuitive at first. If you don't want to be eaten by birds or mantids, why mark your web with a big, conspicuous X? When a St. Andrew's cross spider sits in the middle of the X, aligning her outstretched legs with the arms of the cross, it can make her appear much larger, possibly intimidating potential predators. A spider who feels threatened may also bounce the web up and down, causing both her and the cross to become a blur, which might further frighten or confuse predators.

The cross could protect the spider in other ways, too. Birds who have swooped down to eat these spiders in the past, for example, may learn to avoid this X shape after being blanketed in the hard-to-remove silk.

4. It Doesn't Always Make a Full Cross

There is a high degree of variability in the stabilimenta of St. Andrew's cross spiders. Although some weave big, thick X shapes with all four arms, they are also known to weave an X with anywhere from one to three arms. Sometimes they weave a web without any X at all.

5. Young Spiders Weave a 'Doily'

St. Andrew's cross spiders have a subtler, brownish coloring as juveniles, and they also create a different kind of web decoration. The young spiders do add stabilimenta to their webs, but not in the X shape at first. They start off with a circular design, which the Australian Museum compares to a "silk doily."

This seems to help hide the spiderlings as they sit in their webs, and it might shade them from harsh sunlight, too. As they grow older, they gradually advance from weaving doilies to crosses.

6. Mating Can Be Dangerous for Males

St. Andrew's cross spiders are sexually dimorphic. The big, colorful spiders are females, while males are many times smaller and less vivid. Their mating season is summer and fall, when male suitors start by waiting near the top of a female's web, wisely taking a cautious approach to courtship. A female's web often features several suitors at once, some of whom may be missing legs due to past attempts to woo unreceptive females.

Males weave a mating thread in the female's web, then vibrate it in hopes of winning her affection. Males and females both have dual sex organs, with a left and a right, but the male's organ breaks off while mating to form a "mating plug." This can help thwart competition from other males, but it means each spider is limited to two copulations. On top of that, a male and female can only mate if their organs match, left-to-left or right-to-right, and males who court incompatible females may risk life and limb.

7. Pheromones Help Males Find Ms. Right (or Left)

Love may be a dangerous game for male St. Andrew's cross spiders, but their search for a suitable mate is not entirely a leap of faith. While they can't safely get close enough to see if a female is compatible, males seem able to assess a female's compatibility by smelling pheromones in her web, giving them a chance to reconsider before waltzing in. Males who have already mated once show a preference for single-mated females vs. double-mated females, research has found, although there is still some uncertainty involved.

Pheromones may help males identify a female who has only mated once, but they apparently can't reveal whether her remaining sex organ is on the left or right, so males are still gambling when they step into a female's web.

8. They Aren't Dangerous to Humans

The size of a St. Andrew's cross spider might be intimidating, but it poses very little danger to people. Its venom is not highly toxic to humans, and like most spiders, it is generally not aggressive with people.