WARNING! Graphic images of spiders follow - arachnophobes should not proceed.
Spin-lovers - Nephila in Australia
I have no trouble visualising my earliest clear recollection of an encounter with a golden orb-weaving spider! It would have been in 1974 or '75, I was probably 15 years old, and it was just down from the Baha'i Temple in Ingleside, northern Sydney. I and a couple of school friends were crashing through the bush on our first ever bout of orienteering, a sport quite new to Australia at the time and just introduced to the Pittwater High School sports options. Pushing our way through the dense and prickly bush, I suddenly recoiled abruptly on impact with a strong and stickily elastic web - my feet continued their forward progress, perhaps slipping on the sandy soil, but the rest of me went backwards and down, and I landed hard.
I must have already been familiar with these spiders and their webs, as I knew exactly what had happened. And had I not been running at full tilt, I'd probably have seen and avoided the web, as they are large, prominent and glow golden-yellow in the sunshine - hence the common name for this group of spiders.
|A large Australian Emperor dragonfly caught in the web of an |
Australian Golden Orb-weaving Spider, Nephila edulis.
Apart from catching out unwary humans, the webs of these spiders are strong enough to ensnare very large insects and even small birds and bats. The photo above, taken at Simpsons Gap west of Alice Springs in May 2010, was the first I'd taken of an Australian Emperor dragonfly and I was a little put out (and humbled) that a spider had got to it first.
Nephila, the genus that comprises the golden orb-weaving spiders, occurs in three main, essentially tropical regions around the world. There are two species in the Americas, from the southern US to Brazil and Argentina, including Nephila clavipes, one of the best-studied spiders in the world; 6-8 species from Africa, ranging from the southern tip of the Arabian Peninsula to Madagascar; and there are about 16 species in the Indo-Australasian region, from India and China through to as far east as Tonga and Samoa. Australia (including its dependencies Cocos (Keeling) Islands, Christmas Island, Norfolk and Lord Howe Islands) is home to four species, though only three of these occur on mainland Australia.
Incidentally, the genus name Nephila, is derived from the Greek words nen = to spin, and philos = love - hence 'love to spin' or 'fond of spinning', or as further derived (or corrupted) for the purposes of the title for this post - 'spin-lovers'.
|The spinnerets are clearly visible towards the ventral tip of the abdomen on this |
Australian Golden Orb-weaving Spider (Bowra, Queensland, April 2014).
The three species of Nephila occurring in Australia are:
- Nephila pilipes - Giant Golden Orb-weaving Spider,
- Nephila plumipes - Humped Golden Orb-weaving Spider
- Nephila edulis - Australian Golden Orb-weaving Spider
The Giant Golden Orb-weaving Spider occurs up the east coast of Australia, north from about Bellingen, and across the Top End. It also occurs widely throughout the New Guinea region and as far east as Vanuatu, on Christmas Island, and through much of South-East Asia and as far north as China. It is generally found in rainforest or other densely vegetated situations.
Compared to the other Australian species, the abdomen of N. pilipes is quite elongated, the sternum is usually entirely black, the palps are bright yellow or reddish, and the yellow patches on the legs are restricted to what look like enamel spots on the underside of the main joints.
|The Giant Golden Orb-weaving Spider Nephila pilipes - |
this one at Fogg Dam, east of Darwin in the Northern Territory, June 2008.
|Another Giant Golden Orb-weaving Spider, this one with at least three tiny yellowish males |
in the web around her. Litchfield National Park, south of Darwin, June 2008.
In this genus, the males are much smaller that the females, up to an order of magnitude so (in linear dimensions), and they may be as little as 1%-5% of the weight of a fecund adult female. So it's not surprising that spiders of this genus have been the subject of much study, particularly the evolution of sexual size dimorphism and of their reproductive biology and behaviour more generally.
The comparatively tiny males tend to live in the female's web as well - up to about six males per web - a dangerous way to live as they are apt to be subjected to sexual or copulatory cannibalism, a not uncommon trait amongst various groups of spiders. Apparently there is also a fair degree of size variability between males, and they compete for the sexual favours of the female, who is not at all averse to accepting the (careful) advances of multiple males. For the males, there is a play off between being larger (and thereby being more successful in seeing off the smaller males from the centre of the web where mating takes place) and being smaller (and thus less likely to be cannibalised, either before, during or after the event, by the female).
The Humped Golden Orb-weaving Spider Nephila plumipes occurs predominantly along the eastern coast of Australia, from Cape York to the southern coast of NSW, but also across the Top End and through many of the Pacific Islands. Particularly in the southern part of the range it is strongly associated with mangrove habitats, but it is also found in other coastal situations and may even be found, usually sporadically or at low density, up to several hundred kilometres inland. They are common on islands, including those of the Great Barrier Reef.
|Humped Golden Orb-weaving Spider Nephila plumipes, Currarong NSW, January 2008.|
|Humped Golden Orb-weaving Spider Nephila plumipes, |
Lady Musgrave Island, Great Barrier Reef, Queensland, January 2014.
The Australian Golden Orb-weaving Spider Nephila edulis is the most widespread and probably most common of the three Australian species. It occurs throughout Australia, including in arid regions, tropical savannas, subtropical and temperate coastal areas, and subalpine habitats. It is the only Nephila species in Tasmania, and also occurs to a lesser degree in New Zealand, New Caledonia, New Guinea and perhaps in Java.
Although often very common, it also tends to be more transient, particularly in more arid regions, where populations may be very dense one year and then virtually disappear for several years.
|Australian Golden Orb-weaving Spider Nephila edulis, Uriarra Crossing, ACT, March 2005.|
|Australian Golden Orb-weaving Spider Nephila edulis, Weddin Mountain, NSW, March 2005.|
|Australian Golden Orb-weaving Spider Nephila edulis, Bowra, Queensland, April 2014.|
|Australian Golden Orb-weaving Spider Nephila edulis, Simpsons Gap, NT, May 2010.|
Nephila webs characteristically have a line of silk-wrapped food items strung out above the hub, or centre of the web. This is known as a food cache, and is used during food shortages. There may be as many as 15 items in the cache, depending on need and availability of prey.
But not all prey is necessarily eaten. While prey items that are eaten tend to be fairly large and frequently include grasshoppers, flies, beetles, butterflies, and even dragonflies; some insects such as vespid wasps and a range of unpalatable insects may be avoided or actually removed from the web.
Smaller insects may also be eaten, but it seems there are usually enough to support another type of small spider that also often inhabits the webs as a kleptoparasite. It is a spider from the family Theridiidae (a very diverse family which includes the redback) called Argyrodes and there are species that occur in the webs of virtually all Nephila species worldwide, including these three Australian species. I'll have to keep my eye out as so far I haven't noticed them.
There was a review of the Nephila genus in Australasia done in the mid-2000s by Mark Harvey, Andrew Austin and Mark Adams. The comprehensive paper, The systematics and biology of the spider genus Nephila (Araneae:Nephilidae) in the Australasian region, published in Invertebrate Systematics, is available as a pdf from http://www.publish.csiro.au/paper/IS05016.htm. It's well worth a look.