The saga of the golden orb weaver egg sac

I am waiting eagerly to see if the golden orb weavers emerge again this season. It's a real mystery what is going on. The summer of 2011 brought an influx of golden orb weavers such as the town had never seen before. Last year, there were none. As spring is springing, the obvious question is: will they return? And why weren't they here last year? One theory is given below. This year will tell if it's right! And the other mystery - how did the daddy long-legs know about the feast on offer?

First, a recap. When we moved to Castlemaine in Central Victoria, Australia, in April 2011, there was a glorious adult female golden orb weaver (Nephila edulis, family Nephilidae) at the front door to greet us. So she was named Welcome.

(click to enlarge)

With a body length of over 2 cm, she was huge. Not long after, Welcome produced a golden egg sac in the bushes nearby. It was already well into winter.

The first frost came and the morning sight was welcome dropping from her web. She recovered in the day's warmth, but the next day's frost killed her.

I watched that egg sac, checking it every day. Nothing happened throughout the whole winter. Up in Kalimna Park, where I was watching another half dozen Nephila, three made egg sacs, and all disappeared with the frosts. 

Now, Welcome had spun her egg sac in the wire on a garden arch which was in the wrong place for my garden design. To move it would have wrenched the egg sac from the plants, so I carefully cut the wire and placed the egg sac on the little front porch. Didn't think that through, did I? When the young hatched, they would use the breeze to disperse - and there's only one way the breeze could come into the enclosed front porch - from the front, carrying them nicely straight into the living room. This is what happened:

The egg sac had faded when I cut it lose in September 2011 and placed it on the porch. It was not until early January, in the summer heat, that I noticed a living bundle near it. Young, hundreds of them. I tried counting from the image below, but gave up.

Over the next few days, they spread and regrouped continuously, little white bodies appearing in the mass of brown as a mess of tiny shed skins appeared below. They were moulting, the slightly larger freshly moulted young showing off their beautifully marked white abdomens - only visible when I enlarged the photographs.

At the end of the week, they were well spread, never to regroup again.

A day with a light breeze and they dispersed on their tiny filaments of silk. It was then I realised that I was an idiot. They could only be blown into the house as a breeze doesn't come from the house out into the world. And they did - hundreds of them into the living room. All over the furniture. All over the ceiling. While they were still in groups, I collected them on brooms and other soft objects, leaving them outside to disperse again. But they spread, meaning it was a matter of collecting them one at a time. They were all over ornaments, such as this little one on the cage of a music box with toy songbirds.

But I was not fast enough. The greatest mystyery of all: within a few hours, five daddy long-legs spiders (Pholcus phalangioides, family Pholcidae) had gathered in the front corner of the lounge room where most of the young landed. I have no idea where they came from. We have a lot of daddy long-legs, but there were none in the living room that I had noticed. The daddy long-legs were having a feast. How did they know to come?

Spiders never cease to amaze and entertain me.

So now I wait. None of these young grew to adults during that summer. Despite the huge numbers in autumn 2011, there were none last autumn (March / April in Australia). Will any of them appear this season? 

Note on the confusing daddy long-legs critters:

An American reader has asked about the daddy long-legs in the photo above. Is it a spider or a non-spider harvestman?

Two different groups of animals are called daddy long-legs, the spider version and the harvestman version. Both have eight long spindly legs. Both are arachnids. I gather Americans are more likely to call the spider version 'cellar spiders'. 

The differences: spiders have two body segments with the legs coming from the front bit, the cephalothorax. Harvestmen only have one body segment.  The spider version make webs while the harvestmen don't. Spiders are much more likely to be inside the house than harvestmen. I photographed huge harvestmen in Texas.

If you look at the image on the blog, you'll see the daddy long-legs has two body segments while the harvestman above has only one. Because of the confusion, arachnologists usually refer to the spiders as pholcids and don't use the 'daddy long-legs' common name.

References:

A great reference for Australian spiders is being developed at Arachne.org.au:

http://arachne.org.au/

This site is a working draft for "A field Guide to the Spiders of Australia" by Robert Whyte and Dr Greg Anderson for CSIRO Publishing due to be published in 2014-15.

I'll be using it as my prime site for classification from now on.

Postscript: Greg Anderson has sent me a fascinating paper on Nephila. It seems the questions I am asking about N. edulis are not answered - yet! Plenty of research to do which can be done by ordinary people out there watching them. 

"Given the large number of studies that have focused on the behaviour and evolutionary biology of Nephila spp., it is some- what surprising that little work has been undertaken on their population biology. In the Sydney region, adult female N. plumipes are univoltine, appearing in late December to February (depending on the year), and reproduce from late summer to autumn (February–May) (Austin and Anderson 1978; Elgar et al. 2003a). From June to September (winter–early spring) the population comprises a range of immature size classes, and a declining number of adult females that have all disappeared by August–September. This pattern becomes more complex in the northern, tropical part of the distribution of N. plumipes, where the breeding season is more prolonged, and populations may be bivoltine (A. Austin, personal observation).

Far less is known about the population biology of N. edulis, which occurs throughout the arid interior of the continent (Fig. 87). In arid regions this species appears to have more tran- sient populations, where dense aggregations of adult females will occur at a particular site in one autumn and then disappear, often for many years (A. Austin, personal observation)."

Reference: Harvey, Mark S., Austin, Andrew D., & Adams, Mark. The systematics and biology of the spider genus Nephila (Araneae: Nephilidae) in the Australasian region. 

Invertebrate Systematics, 2007, 21, 407–451