The spider that weaves gold
Article | Updated 6 months ago
Often mistaken for a dangerous creature, the Australian golden orb-weaving spider is in fact harmless to humans. They have a dark-brown carapace (the “head”), a cream coloured abdomen, and yellow banded legs. Males are very small, and are often difficult to locate in the web of the female. They occur all over Australia, building large webs with yellow silk which shine like gold in sunlight.
Taxonomists work on the classification of living organisms, describing, identifying and naming each newly discovered species. This scientific work leads to a precise classification subdivided into several entities called “taxa”; these include kingdom, class, order, family, genus and species. Let’s take a look at the taxonomy of the Australian golden orb-weaving spider.
|Taxa||Australian Golden Orb-Weaving Spider|
|Common name:||Australian golden orb-weaving spider|
Although they are often mysterious, scientific names are not randomly given. Thus, when the French biologist Jacques Labillardière travelled to New Caledonia in 1799, he applied the name edulis, which means “edible”, to the Australian golden orb-weaving spider after he observed native people eating this spider!
A web of golden threads
Nephila edulis construct large orb-webs that are between 0.5 and 1 meter in diameter, using a yellow-coloured silk leading to the common name “golden orb-weaving spider”. Australian golden orb-weaving spiders remain permanently in their webs, which are themselves permanent structures that function as a prey-catching device during both the day and night. As a consequence both juveniles and adult females are possibly more exposed to diurnal hunting predators over relatively long periods of time, and their webs are exposed to damage from birds, grazing mammals and large insects. Several physical and behavioural adaptations have resulted to minimise these effects. Firstly, the webs are protected by a strong “barrier” web on one or both sides, a kind of extensive random network made by plant detritus and insect carcasses clumped with silk. This “barrier” web may protect them from large aerial predators such as birds. Furthermore, females often aggregate with each other so that their webs become interconnected, further reducing predation. However, the results of studies to examine the function of this behaviour are contradictory. While some studies indicate that aggregating behaviour has positive benefits for prey capture efficiency and reduces aerial predation, other results suggest “web clumping” is a random process influenced by population density and results in decreased predation success.
Like most diurnal orb-weavers, Nephila edulis are opportunistic, feeding on any suitable prey that adheres to their web. However, they are sometimes selective predators and avoid or remove particular insects from their webs without feeding on them. These include some types of wasps, winged ants, “unpalatable” butterflies, and numerous “obnoxious” groups including some beetles that produce distasteful secretions.
Different hunting techniques for different types of prey
Nephila edulis females are able to deal with a large size range of prey, from around 2 mm in length up to insects larger and heavier than themselves. Small birds and bats are very occasionally caught in the web but it is not known if they are eaten by the host.
When a prey adheres to their web, Australian golden orb-weaving spiders orient and approach the prey with web vibrations and adapt their attack behaviour according to the prey’s size. Small prey are simply “seized and removed” from the web, while large prey are bitten by the resident spider that then waits for the venom to subdue the prey before it is cut out of the web and wrapped in silk.
Australian golden orb-weaving spiders also construct food caches, usually above the hub of the web and utilised when there is a shortage of food. Caches vary in size and composition and are influenced by prey density, encounter rates and prey type. Large caches may contain as many as 12–15 prey items and may be 10–12 cm in length. When prey is added to the cache it is usually densely wrapped in silk and placed at either the top or bottom of the cache so the connected prey forms a near vertical line. The dense silk covering probably helps to reduce dehydration of the prey, but it and the cache may also have other functions. Recently, studies on the food caches of Nephila edulis, show that they are constructed and expanded during periods of high prey availability, and that access to a cache reduces weight loss, particularly for large females, at times of prey scarcity.
A dwarf male or a giant female? Scientists disagree
Males Nephila edulis are much smaller than females. Whereas females can grow up to 2 or 4cm, males can reach a maximum size of 6mm. This extreme size difference due to gender, called “sexual dimorphism”, has generated debate in scientific community on whether the dimorphism has evolved through giant female or dwarf males.
Mating has never been so dangerous!
For mating, the male cautiously approaches the female, sometimes plucking a radial line. If the female is receptive the male will move to a position on the female’s ventral abdomen, facing forwards, and begin mating. If the female reacts aggressively to the presence of an approaching male, he will first become motionless, often for 10 to 15 minutes, and if she continues to be aggressive when the male resumes an approach, he moves away from the female.
Sexual cannibalism is generally uncommon in orb-weaving spiders, but may occur. Several behavioural adaptations in males have been identified that act to reduce sexual cannibalism. In particular this includes mating with newly moulted adult females that are largely inactive, and mating when the female is feeding, but also, to a lesser degree, approaching the female on the opposite side of the orb web. Females Nephila edulis generally produce one or several egg sacs containing an average of 380 eggs.
A common spider species
The Australian golden orb-weaving spider occurs in Cocos Keeling Islands, Papua New Guinea, New Caledonia, New Zealand and Australia, including Tasmania.
In Australia Nephila edulis is commonly found all around the country, in open forests and shrubland, but they also occur in dense vegetation and near small water-courses. The webs of this species are often found between bushes and trees but Australian golden orb-weaving spiders sometimes construct their webs against buildings and other structures.
A spider on the menu
Information on the natural enemies of the Australian golden orb-weaving spider and the levels of mortality they cause are restricted to anecdotal observations and there are no detailed studies available. Unlike other spiders, species from the genus Nephila are apparently rarely attacked in their webs by Hymenoptera, such as ants, wasps and bees, which catch and paralyze insects and spiders to feed on their offspring (predatory Hymenoptera), or lay in other arthropods, allowing larvae to feed on the host’s living tissues (parasitic Hymenoptera). Even if they don’t come from the wild, Nephila edulis has some enemies. In South-East Asia and the Pacific people have used it as food source and may, in the past, have reduced the size of populations around villages and settlements.
The Australian golden orb-weaving spider is not aggressive and their bite, causing mild local pain, numbness and swelling, is not lethal for humans. Thus, if you see an Australian golden orb-spider don’t panic and contribute to increase knowledge about this species recording your find on the website of Atlas of Living Australia!
This article is based on a paper published by Mark Harvey, Head of Department Arachnids and Myriapods at the Western Australian Museum, about the Australian golden orb-weaving spider and others Nephila species. Read the full paper here.
Find out more information and photos of Australian golden orb-weaving spiders on OzAnimals website.