Have you noticed the unusually high population of golden orb-weaving spiders (Nephila edulis) in Melbourne this year? They're usually very rare this far south but I’ve spotted dozens of them in the inner-city suburbs over recent months. Our online visitors have too; in the past three months, we’ve received over 50 comments on this Question of the Week about these spectacular spiders.
Discovery Centre gets a lot of queries about spiders and whether they’re dangerous, often after they’ve received a lethal dose of insect spray, so it’s delightful to see that most of the recent comments simply marvel at the size, beauty and architectural skills of these spiders. Lots of people have told us they are quite fond of their backyard Nephila and some have even given them names! We’ve heard about Bertha, Gloria, Holly, and, I confess, I’ve named the one that lives near me Nefertiti.
Nefertiti the large female Nephila edulis.
Source: Museum Victoria
Because people are so interested, I thought I’d dig up a bit more about Nephila edulis. They are more often found in northern Victoria, NSW and QLD where there has been a bumper spider season, too. Professor Mark Elgar from the University of Melbourne has studied these spiders for many years, travelling to Euroa each spring to collect specimens for behavioural studies. He recently commented in the Shepparton News that high summer rainfall “has provided a lot more food for flying insects, which become food for spiders. They really are much more abundant than I've seen for a long time and next year we'll see the same thing.”
Nefertiti sits in her large golden web all day, unlike the nocturnal and more common Garden Orb-weaving Spider (Eriophora sp.), which tears down and rebuild its web almost daily. Nefertiti leaves her web up until it’s so ratty that it needs to be repaired and her home is adorned with a rather gruesome array of dead insects. Professor Elgar and his colleagues showed that this vertical band of detris is a stockpile of food but also serves another intriguing function; it attracts more food. The spiders deliberately incorporate bits of rotting vegetation to make their larders irresistable to flies.
The underside of a large mature female Nephila edulis on her web. In the background is her egg sac and hanging in her web is a detrius band of dead insects.
Source: Museum Victoria
Another fascinating aspect of Nephila biology is the difference in size between males and females. While females are generally much larger than the males, within males there is a big variation in size. Professor Elgar and colleagues investigate how this has evolved. It’s a complex question with no definite answers and lots of factors to consider.
A pair of golden orb-weaving spiders illustrating the difference in size between males and females. The tiny male is on the left while the large female, feeding on a moth, is on the right.
Image: Bill & Mark Bell
Source: Used under Creative Commons license (CC BY-NC-SA 2.0) from Bill & Mark Bell
Male N. edulis have two strategies when it comes to approaching a female. The risk of being mistaken for her lunch is pretty high so it pays to be careful. One tactic is to crawl onto the web on the same side as the female, while another is to approach from the opposite side and cut a hole in the web. Small males are more common than large males and they tend to use the first strategy. They also mate for longer and father more of the female’s offspring. However there are costs to being small, too: smaller males are more often eaten by females than large males. Furthermore, if there are a number of males loitering around the edge of a female’s web, large males beat small males in the battle to reach the female.
I don’t know if she was courted by a large or small male (or both - these spiders mate several times), but Nefertiti has laid a clutch of eggs in a golden silk sac. In spring her eggs will hatch and her babies will disperse on the wind to start the whole cycle again. Keep an eye out for them later in the year! Meanwhile, if you’d like to see a golden orb-weaver up close, visit the Orb Wall in Bugs Alive! at Melbourne Museum.
The golden silk egg sac of Nephila edulis.
Source: Museum Victoria
B. T. Bjorkman-Chiswell, M. M. Kulinski, R. L. Muscat, K. A. Nguyen, B. A. Norton, M. R.E. Symonds, G. E. Westhorpe and M. A. Elgar. 2004. Web-building spiders attract prey by storing decaying matter. Naturwissenschaften 91:245-248
J. M. Schneider, M. E. Herberstein, F. C. de Crespigny, S. Ramamurthy and M. A. Elgar. 2000. Sperm competition and small size advantage for males of the golden orb-web spider Nephila edulis. Journal of Evolutionary Biology 13: 939-946