The golden silk orb-weavers (Nephila) are a genus of araneomorph spiders noted for the impressive webs they weave. Nephila consists of numerous species found in warmer regions around the world. They are also commonly called golden orb-weavers, giant wood spiders, or banana spiders.
Etymology
The genus name Nephila is derived from Ancient Greek, meaning "fond of spinning", from the words νεῖν (nein) = to spin (related to nema νήμα "thread") + φίλος (philos) = "love".
Description
Nephila spiders vary from reddish to greenish yellow in color with distinctive whiteness on the cephalothorax and the beginning of the abdomen. Like many species of the superfamily Araneoidea, they have striped legs specialized for weaving (where their tips point inward, rather than outward as is the case with many wandering spiders). Their contrast of dark brown/black and green/yellow allows warning and repelling of potential predators to whom their venom might be of little danger.
Golden orb-weavers reach sizes of 4.8â€"5.1 cm (1.5â€"2 in) in females, not including legspan, with males being usually 2/3 smaller (less than 2.5 cm, 1 in). The largest specimen ever recorded was a 6.9 cm (2.7 in) female N. plumipes (which is now debated to have been a new yet undocumented subspecies) from Queensland, Australia, that was able to catch and feed on a small finch. In 2012 a large individual was photographed killing and consuming a half-metre-long brown tree snake in Freshwater, Queensland. Species from Taiwan have been known to reach over 130 mm (5.1 in), legspan included, in mountainous country. In 2014, a study discovered that golden orb-weavers living in urban areas, particularly those of a high socioeconomic status, grew larger and carried more eggs than those in their native habitats. A number of possible explanations were suggested, such as increased food supplies due to artificial light or lack of predators and parasites.
Species
As of May 2017, the World Spider Catalog accepted the following species:
Distribution and habitat
Golden silk orb-weavers are widespread in warmer regions throughout the world, with species in Australia, Asia, Africa (including Madagascar), and the Americas. One species, N. clavipes, occurs in the United States of America, where it ranges throughout the coastal southeast and inland, from North Carolina to Texas. Spiderlings can be carried by the wind over long distances, and each year, a small number of golden orb web spiders are found in New Zealand (where they are not endemic) after having been blown across the Tasman Sea; the spiders usually end up in the North Island.
Whilst the geographic distribution of Nephila is large, there are many habitat similarities between these locations. A warm and reasonably wet climate is generally preferred, as these are some of the environmental cues that induce spiderling hatching. Locally, spiders look for relatively dense vegetation where webs can be set up in areas that insects will regularly fly through. Urban environments are also attractive due to the large prey concentrations and lower amounts of predation.
Behavior
Web spinning and structure
Nephila produce large asymmetric orb webs up to one and a half meters in diameter. The hub of the web is not positioned centrally but rather in the upper section; this means that the majority of the sticky capture strands are found in the lower section of the web.Nephila species remain in their webs permanently and therefore have a higher predation risk. There is a barrier web structure on either side of the main web to mitigate this risk. The golden silk orb weaver is named for the yellow color of the spider silk used to construct these webs.
Yellow threads of their web shine like gold in sunlight. Xanthurenic acid, two quinones and an unknown fourth compound contribute to the yellow color. Experimental evidence suggests that the silk's color may serve a dual purpose: sunlit webs ensnare bees that are attracted to the bright yellow strands, whereas in shady spots the yellow blends in with background foliage to act as a camouflage. The spider is able to adjust pigment intensity relative to background light levels and color; the range of spectral reflectance is specifically adapted to insect vision.
The webs of most Nephila spiders are complex, with a fine-meshed orb suspended in a maze of non-sticky barrier webs. As with many weavers of sticky spirals, the orb is renewed regularly if not daily, apparently because the stickiness of the orb declines with age. When weather is good (and no rain has damaged the orb web), subadult and adult Nephila often rebuild only a portion of the web. The spider will remove and consume the portion to be replaced, build new radial elements, then spin the new spirals. This partial orb renewal is distinct from other orb-weaving spiders that usually replace the entire orb web. In 2011 it was discovered that the web of Nephila antipodiana contains ant-repellent chemicals to protect the web.
Typically, the golden orb-weaver first weaves a non-sticky spiral with space for two to twenty more spirals in between (the density of sticky spiral strands decreases with increasing spider size). When she has completed the coarse weaving, she returns and fills in the gaps. Whereas most orb-weaving spiders remove the non-sticky spiral when spinning the sticky spiral, Nephila leave it. This produces a "manuscript paper" effect when the orb is seen in the sun: groups of sticky spirals reflecting light with "gaps" where the non-sticky spiral does not reflect the light.
The circular-orb portion of a mature N. clavipes web can be more than 1 meter across, with support strands extending perhaps many more feet away. In relation to the ground, the webs of adults may be woven anywhere from eye-level upwards high into the tree canopy. The orb web is usually truncated by a top horizontal support strand, giving it an incomplete look.
Adjacent to one face of the main orb there may be a rather extensive and haphazard-looking network of guard-strands suspended a few inches distant across a free-space. This network is often decorated with a lumpy string or two of plant detritus and insect carcasses clumped with silk. This "barrier web" may function as a kind of early-warning system for incoming prey or against spider-hunting predators, or as a shield against windblown leaves; it may also be remnants of the owner's previous web. At least one reference explains the suspended debris-chain as a cue for birds to avoid blundering into and destroying the web.
Stabilimenta among N. clavipes are sometimes seen in the webs of immatures nearing molt, hence the names "molting webs" or "skeleton webs" (webs with radial strands but no spiral elements).
Prey capture and feeding
The golden silk orb weaver targets many different organisms as prey, ranging from small flies and beetles to larger cicadas and locusts. As a result of their strong web structure small birds and bats can also become trapped and fed upon. Whilst most of the captured prey is relatively small compared to Nephila sp the majority of biomass consumed comes from larger more rare prey. Prey larger than 66% of the captors size accounts for just 16.5% of prey captured but 85% of prey consumed, indicating the spider is selective in its feeding habits.
Throughout the maturation process spiders exhibit different feeding behaviours relative to their web type. Originally after birth the spiderlings inhabit a communal web and eat dead siblings and dead prey but do not attack any live organisms. After they move to individual webs the spiderlings begin to attack small live prey. As they increase in size they become more ambitious in their prey capture attempts through larger and stronger webs and greater attack confidence.
Spiders are notified that potential prey has been caught in the web through vibrations along strands, these can be followed to the prey location on the web and be used to estimate prey size. Due to the large variety in prey Nephila has different attack techniques dependent on prey size. Small prey like flies are captured alive then removed from the web and wrapped in silk. This occurs with smaller prey because they do not present a threat to the spider when alive. Prey that is seen as potentially threatening will be bitten to be subdued before wrapping. Medium-sized prey will receive bites that are sustained until prey is immobilised and then be covered in silk. Finally, the largest captured prey receive the â€"bite and back offâ€" treatment where a single lunging bite is made before the spider withdraws and waits for the venom to take effect before wrapping. This lunging bite is often repeated to speed up the process of immobilisation.
Nephila also create caches of food for storage, these can be found above the hub of the web and contain up to 15 prey items. These items are arranged in a line vertically and are wrapped in silk to reduce dehydration. Caches are created and grow when prey is readily available and there is more biomass available for consumption than is required by the spider. The purpose of caches is to have a backup food source when prey is scarce and occasionally to provide bait to attract more prey to the web. Nephila may also respond to food shortages by moving their webs, however this is a response to longer periods prey scarcity than cache creation. Web moving is seen as a result of environmental change whereas caches occur from environmental fluctuation.
Mating and reproduction
Nephila display large sexual size dimorphism with females being greatly larger than males, sometimes up to ten times the size. There is debate as to whether this is a result of male dwarfism or female gigantism. Smaller males may be selected for due to the presence of competition for mating. Smaller males are quicker and more nimble, allowing them to be able to catch the females more easily as well as escape when threatened. Larger males may have to wait for the female to come close due to their slower speed. Larger females may have been selected for as a result of males using mating plugs upon copulation. Larger individuals reduce the success of these plugs, allowing for multiple mating and reducing the risk of genital mutilation. Gigantism in females is also associated with fecundity, larger individuals can produce more eggs and therefore increase reproductive success.
When males are fully mature they leave their webs to search for a suitable female, often using web characteristics to identify potential mates. Females are most likely to mate directly after their final molt at the beginning of adulthood and later in adulthood when they are feeding. There are often multiple males attempting to court the same female and thus competition for territory on the web occurs but is rarely physical as smaller males surrender area to larger ones. When males approach females they are often feeding, allowing the males to get closer without an aggressive response and also meaning the female is not moving. On approach the male makes himself known by tapping on a web strand to ensure the female is amenable before proceeding to mate. When met with aggression males will stop approaching and remain in the same location until the female relaxes or they retreat. Females engage in multiple mating but there is no benefit to the offspring as a result of this, however the energy cost of repelling a male is higher than that of allowing him to copulate. As a result of this sperm competition occurs through males altering the duration of mating as well as the frequency, longer mating is proportional to a greater likelihood of success. Sexual cannibalism is uncommon in Nephila as a result of male mating behaviours. By copulating when females are immobile after molting or inactive due to feeding the males increase their chances of survival. Males also approach on the opposite side of the web to the female making it more difficult to be caught. Sexual cannibalism does still occur and generally is more common in larger males and from older females.
Female Nephila produce an egg-sac in the surrounding environs of the web to protect their eggs. The eggs are deposited on a silk platform then are covered in loose silk to form a sac. This sac is firmly attached to surrounding vegetation so that it is hidden from view of predators. These sacs can contain from 300 to 3000 eggs depending on the mating success and species. Once hatched the spiderlings will inhabit a communal web to begin their lives.
Thermoregulation
Nephila change their body positioning relative to the sun to maintain internal temperatures at an optimal level. As ambient temperatures increase the spider will position itself so the abdomen shades the cephalothorax from the sun. Spiders may also hang from their hind legs as a result of the heat due to a loss of hydrostatic pressure. Conversely as temperatures cool down the spider will position itself perpendicular to the sun to retain as much heat energy as possible. When ambient temperatures reach extreme highs (above 40 °C) Nephila may leave their web and seek shade in the surrounding environment.
Predation and parasitism
Predation of Nephila is relatively uncommon, when it does occur the main group affected are the juvenile individuals. The major predators are birds but wasps and damselflies also prey upon smaller juveniles. To combat this females aggregate their webs to form a large complex, discouraging aerial predators for fear of entanglement.
N. clavipes (and many other Nephila species) are frequently parasitized by Argyrodes, a genus of very small black-and-silver spiders that are kleptoparasitic. As many as a few dozen may infest a single Nephila web to feed from the host spider's captured prey. The frequent rebuilding or abandoning of webs by Nephila may be a tactic for controlling Argyrodes. Spiny orb-weaver spiders of the genus Gasteracantha also inhabit the webs of Nephila as kleptoparasites, too.
Egg-sacs generally remain free from both predation and parasitism, often due to the close proximity of the mother and how well it is hidden. Moth larvae of genus Anatrachyntis have been known to prey on Nephila eggs but this is the only well known and researched case.
Life cycle
Spiderlings
Nephila spiderlings leave the egg-sac as a result of environmental cues, often warmer and wetter conditions in spring. They then live on a communal web eating dead siblings and web debris for around a week before dispersing to make individual webs.
Young spiders do not generally build yellow-colored silk, and the young Nephila themselves can be easily mistaken for young Orchard Spiders (Leucauge) in general color and shape (both genera sport silver stripes or patches on their abdomens, described in some references as a form of heat control). The best distinction between Leucauge and Nephila juveniles is web structure: Leucauge tends to build a horizontal orb that is a perfect circle, whereas Nephila build vertical, elliptical orbs that are incomplete (missing the portion of the orb over the hub, the center where the spider sits). Nephila seem to prefer more open habitat such as second-growth scrub or forest edges. Fences or building overhangs often do just as nicely.
In addition, young spiders demonstrate vibrational motion when approached by a predator. They will oscillate at approximately 40 Hz when the web is pluckedâ€"thought to be a response to a potential predator. If a predator persists in an attack, the spider will either run to a web-support strand and thus to nearby vegetation, bail out of the web on a silk line that remains connected to the web, or jump from the web after inducing oscillations in the web that aid the jump.
Molting and growth
Once they are juvenile, Nephila inhabit their individual webs then begin the growth by molting process. The period of time between molts is called an instar and there can be 7-12 of these depending on food availability. Ecdysis, or the shedding of the exoskeleton occurs through the formation of a soft exoskeleton inside the current one. Once the old exoskeleton is shed the new larger one begins to harden. Ecdysis occurs when the spiders mass becomes too great for the current exoskeleton to support. Once they are mature male spiders seek out females for copulation and live on their webs. When mating season arrives both males and females stop molting and remain the same size for the remainder of their lives.
Venom
The venom of the golden silk orb-weaver is effective in action on prey, but has not been reported to be of any notable consequence for humans if accidentally bitten. In the literature, Nephila is one of several genera where the venom "must be considered as more or less ineffectual in human beings". That said, the potentially large size of several members of the genus means that they possesses relatively strong chelicerae, so obviously any bite can cause some mechanical damage, but for humans of only short-term localised effect. However, active components of the venom in some Nephila have been shown to be insecticidal, most notably containing some specific blockers of glutamate receptors (i.e. glutaminergic neurotransmission) in other arthropods, such as NSTX-3 JSTX-3 and Nephilatoxins (NPTX-1˜12),. However, further studies of the venom components are needed to better understand pathways associated with any toxicity.
Interaction with humans
Nephila do not seem to form either beneficial or harmful relationships with humans. Females often construct their webs using human structures as a base for support strands due to their stability. Individuals are often found in urban and suburban environments due to the protection from predation and greater prey availability. As they weave their webs in bushes and near flowers, they might present a nuisance for gardeners or flower pickers. Some nests near fruits may repel or destroy known pests, such as Tephritid fruit flies, without the need to use insecticides. Spiders may bite humans if provoked but more often flee if confronted.
Golden silk
There have been several efforts in the past to produce garments from Nephila silk although none commercially viable. These include two bed hangings that were shown at the 1900 Paris Exhibition. In 2004 a textile designer, Simon Peers, and an entrepreneur, Nicholas Godley managed in three years work and using 1.2 million Golden silk orb-weavers (collected in the wild and released some 30 minutes later after they produced the silk) to produce a shawl that was as exhibited at the American Museum of Natural History in 2009. By 2012 they managed to produce a second, bigger garment, a cape, that, together with the shawl, were exhibited at the Victoria and Albert Museum in London.
Another possible use of Nephila silk lies in tissue engineering. A study from the Medizinische Hochschule Hannover reports that processed Nephila silk is an excellent scaffold material thanks to its biocompatibility, mechanical strengths, and its property to promote cell adhesion and proliferation. In particular, the silk acts as a suitable guiding material for peripheral nerve regrowth.
Fishermen on coasts of the indopacific ocean remove Nephila webs and form them into a ball, which is thrown into the water. There it unfolds and is used to catch bait fish.
References
External links
- Description, distribution and picture of N. plumipes
- Nephila clavipes - golden silk spider on the UF Entomology department website
- Studies of the golden silk spider, weaving and catching prey
- Australian Nephila edulis
- Pictures of golden silk orb-weavers
