Nephila is a genus of araneomorph spiders noted for the impressive webs they weave. Nephila consists of numerous species found in warmer regions around the world, although some species formerly included in the genus have been moved to Trichonephila. They are commonly called golden silk orb-weavers, 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, most of them 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 which their venom might be of little danger.
Golden orb-weavers reach sizes of in females, not including legspan, with males being usually two-thirds smaller (less than 2.5 cm, 1 in). In 2012, a large individual was photographed killing and consuming a 0.5-m-long brown tree snake in Freshwater, Queensland. Species from Taiwan have been known to reach over , legspan included, in mountainous country. In 2014, a study discovered that golden orb-weavers living in urban areas, particularly areas 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 In 2018, twelve Nephila species were reclassified as Trichonephila, with another two (N. kuhlii and N. robusta) considered in 2020 to be junior synonyms of N. pilipes. Species whose placement has been changed by some sources include: Nephila laurinae was considered to be a synonym of Trichonephila antipodiana ("Batik Golden Web Spider"), found most commonly in the Philippines and Vietnam Trichonephila komaci, found most commonly in South Africa and Madagascar, transferred from Nephila komaci Trichonephila clavipes (or "banana spider") found widely in warmer parts of the Americas, transferred from Nephila clavipes Trichonephila inaurata (or "red-legged golden orb-weaver spider") found most commonly in southern and East Africa, transferred from Nephila inaurata Trichonephila plumipes (or "tiger spider") found most commonly in Australia, transferred from Nephila plumipes
, the World Spider Catalog does not accept all of these changes, listing the following species in the genus Nephila: Nephila comorana Strand, 1916 – Comoros, Mayotte Nephila constricta Karsch, 1879 – Tropical Africa Nephila cornuta (Pallas, 1772) – Guyana Nephila dirangensis Biswas & Biswas, 2006 – India Nephila kuhli (Doleschall, 1859) – Myanmar to Indonesia Nephila pakistaniensis Ghafoor & Beg, 2002 – Pakistan Nephila pilipes (Fabricius, 1793) (type) – India to China, Vietnam, Philippines, Taiwan, Australia Nephila tetragnathoides (Walckenaer, 1841) – Fiji, Tonga, Niue Nephila vitiana (Walckenaer, 1847) – Indonesia, Fiji, Tonga
Additional fossil species are known from the Cenozoic. In 2012 Geratonephila burmanica was described from the Cenomanian aged Burmese amber, Wunderlich 2015 synonymised Geratonephilia with Nephilia tenuis, a species from the Dominican Amber, as he considered it unlikely that the amber was actually Burmese in origin. Though largely ambiguous, the origins of Nephila are undoubtedly Gondowanan. With the prime candidates being Africa, Indomalaya, and Australasia.
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. Spiderlings can be carried by the wind over long distances, and each year, a small number of female 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, many habitat similarities are seen between these locations. A warm and reasonably wet climate is generally preferred, as these are some of the environmental cues that induce spiderling hatching.Higgins, L. (2017). Nephila Life Cycle. [online] Uvm.edu. Available at: http://www.uvm.edu/~lehiggin/LifeCycle.html [Accessed 30 May 2017]. 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 levels of predation.
Behavior Web spinning and structure Nephila spiders produce large asymmetric orb webs up to in diameter. Nephila species remain in their webs permanently, so have a higher predation 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. Carotenoids are the main contributors to this yellow color, but xanthurenic acid, two quinones, and an unknown compound may also aid in the 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), subadults and adults often rebuild only a portion of the web. The spider removes and consumes the portion to be replaced, builds new radial elements, then spins the new spirals. This partial orb renewal is distinct from other orb-weaving spiders that usually replace the entire orb web. The web of Nephila antipodiana contains ant-repellent chemicals to protect the web.
Typically, the golden orb-weaver first weaves a nonsticky spiral with space for two to 20 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 nonsticky spiral when spinning the sticky spiral, Nephila spiders 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 nonsticky spiral does not reflect the light.
In relation to the ground, the webs of adults may be woven 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, a rather extensive and haphazard-looking network of guard-strands may be suspended a few centimeters 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.
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, the majority of biomass consumed comes from larger, rarer prey. Prey larger than 66% of the captor's size accounts for just 16.5% of prey captured, but 85% of prey consumed, indicating the spider is selective in its feeding habits.
Spiders are notified that potential prey has been caught in the web through vibrations along strands, and these can be followed to the prey location on the web and be used to estimate prey size.
Nephila species also create caches of food for storage, which 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 more biomass is 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 species may also respond to food shortages by moving their webs, but this is a response to longer periods of prey scarcity than cache creation.Robinson, M. and Robinson, B. (1973). Ecology and behavior of the giant wood spider Nephila maculata (Fabricius) in New Guinea. Smithsonian Contributions to Zoology, (149), pp.1-76. Web moving is seen as a result of environmental change, whereas caches occur from environmental fluctuation.
Mating and reproduction
Nephila spiders display large sexual dimorphism in size, with females being greatly larger than males. Debate exists 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 to 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, as larger individuals can produce more eggs and therefore increase reproductive success.