Golden Orb Web Spiders

Spiders: belong to the class Araneae; they are among the best known and most widely distributed of all the arachnids, comprising about 34,000 species distributed into approximately 100 families worldwide. Its members have conquered, with perhaps the exception of the open sea, all ecological environments including one at 23,000 ft on Mount Everest. Despite the fact that none can breathe in water, one species from Europe, Argyroneta aquatica, is totally aquatic.

Their body is divided into two major parts (cephalothorax: the first anterior body division composed of the head and thorax fused into one structure; and abdomen: the second, posterior body division) with a deep constriction between these (pedicel); they lack antennae and typically have eight legs. Most spiders are relatively small (2-10mm body length), although some large “tarantulas” may reach a body length of 80-90mm.

Their most characteristic feature is the ability to produce, through spinning glands, silken threads used for making egg sacs (cocoons), draglines and for building snares. Many spiders are specialized as snare builders (web spiders). However, other spiders do not construct webs to catch their prey but actively chase them instead (wandering spiders).

Orb weavers: Families Araneidae and Tetragnathidae are referred to as the orb-web spiders. This is one of the most diverse groups with various shapes, colours and camouflage systems. Orb webs have developed as an efficient means of capturing flying insects, therefore occupy a niche not utilized by other spiders: the aerial passage amongst vegetation in the path of flying insects. Their structure provides a unique combination of large capture area with near invisibility, making detection and avoidance difficult, especially at night. Each species of spider has its own characteristic web design. Webs may be vertical, inclined and rarely horizontal.

There are both diurnal (day) and nocturnal (night) species. Some groups construct a web every day (or night) which is taken down and eaten. This recycling process serves as a valuable source of protein, since it returns the silk protein to the silk glands to make new silk. Many nocturnal spiders, such as Eriophora, eat their web toward dawn and rebuild it each night. Other spiders, for example Nephila, continually repair their webs after being damaged and before being taken down, using them for a number of days.

The genus Nephila: referred as golden orb web spiders, is widely distributed in tropical and temperate regions of the world and is well known for the large (often 60 cm in diameter) and strong yellowish silk web.

Nephila is sexually dimorphic. Mature males can be less than 10% of the size of a mature female, and several males (one to six) may cohabit on the web of both unmatured and mature females. Males are usually found at the top of the web. The female sits at the hub facing downwards waiting for insects that become trapped in the web. Once prey is caught, the spider cautiously approaches the prey, wraps it in silk to immobilize it and kills it with a bite, probably injecting venom and digestive enzymes. The prey is then moved to the centre of the web where it is either consumed or added to the food store. Nephila is active in prey capture both day and night and will take a wide variety of insects ranging from small dipterans to large scarab beetles and hawk-moths.

Nephila edulis: is found in the drier woodland regions of eastern Australia, including both tropical and temperate regions. It is distributed widely throughout Australia, extending from Sydney and central New South Wales to Queensland, including many islands of the Great Barrier Reef, and Western Australia.

The web is approximately 1m in diameter and it is asymmetrical with the hub off-centre, closer to the top of the web. The orb is strengthened by many supporting lines to the surrounding foliage and is often protected on one or both sides by a simple but strong 'barrier' web. The radial and support lines are whitish in colour and the top supporting line and spirals are golden yellow.

The female has a body length of 20-23 mm. The cephalothorax is black with a white pattern dorsally and a yellow sternum. The abdomen is variable in colour and ranges from grey to brown; being light grey in gravid females. There are eight yellow spots dorsally and a number of thin transverse white lines on the ventral surface of the abdomen. The legs are long with a yellow band at each joint. The male is 4-6 mm long and has a brown cephalothorax and abdomen, both with white markings. The sternum is yellow and the legs are long with yellow bands at each joint, as in the female.

N. edulis show that mating is closely associated with feeding. The male always begins its mating sequence after a female has caught prey. If she begins feeding, the male approaches very slowly. When the male reaches the female he stops, sometimes touches her legs, and then moves to her side of the web. Then, he climbs underneath her abdomen and applies his palps alternately to her genital pore. If disturbed, males return quickly to the opposite side and move to the top of the web. Duration of copulation is variable and smaller males generally copulate longer than larger males.

N. edulis breeds during the autumn (February-May), the female produces one cocoon per season, containing an average of 380 eggs, which are laid only at night between small branches and left next to the web. Once the cocoon is completed, the female did not return to it. The prehatching development time (egg and an embryonic period) is long, often several weeks.

Role in habitat: All spiders are an important group of carnivorous predators. Although insects constitute the major source of prey for spiders, certain other arthropods are often consumed as well. Studies have shown that most spiders feed on a wide variety of prey. Comparison of the prey of Nephila species shows that the genus is an opportunistic predator and will eat anything that lands in its web. For instance, N. maculata caught prey belonging to at least eight insect orders; while butterflies, moths, flies, crickets, bees, wasps and beetles have been listed as the largest prey groups of Nephila in Australia.

Although they are harmless to man, one of the main reasons many people kill spiders on sight is the ugly looking web. But the fact is that spiders are beneficial to human beings. What people are not so aware of is that even creatures commonly found in their own homes can play an important role in controlling domestic pests. The more than 2000 orb-weaver species are one such example of a beneficial “house guest”, in the sense that they feed on the pests of man such as cockroaches and flies.

Spiders as biological control agents: Any insect or plant is not ‘alone’ in its environment, but is accompanied by (and may interact strongly with) other species, some of which may be antagonistic to its well-being. Before chemical agriculture and chemical pest control came into the picture, organisms lived in balance with each other in different ecosystems. Today, the kind of agricultural development which is based on monoculture and extensive use of chemicals has led to the emergence of several pests which has in turn led to the failure of crops. The number of pests developing resistance to pesticides over the years has been increasing at a very alarming rate. The pesticides have proven to be extremely toxic and have led to a number of side effects: impact on public health, toxic residues in food and disturbance of local ecosystems.

Total reliance on synthetic chemical pesticides for pest suppression entails many severe and costly health, environmental, and even pest management side effects. Biological control is the use of biological agents (primarily predators, parasitoids and pathogens) that interact with pest insects and other arthropods or pest weeds, and reduce pest numbers and the viability of their populations. Spiders are an important group of biological control agents, since they consume a good amount of insects invading crops, and they are able to survive periods of starvation when prey is difficult to find. Field experiments, performed over the last 35 years, have demonstrated that spiders can reduce insect populations and the crop damage they cause, also improving crop health and productivity. Despite their ubiquity and high densities, spiders have not received the recognition they need in order to be fully utilized as biological control agents.

Current conservation status: the use of several toxic chemicals, often recommended controlling pests in households and crops, has led to the eradication of natural predators such as spiders which normally keep pest populations in balance. Up to date, several species of spiders have been recognized as rare or worthy of concern on three different lists of threatened species in the United States: the Endangered Species Act (ESA) IUCN Red Lists, and lists compiled by The Nature Conservancy and Natural Heritage Programs.

Golden Orb Web Spiders - genus Nephila