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ORBWEB SPIDER

20 July 2012

QUICK FACTS

Scientific Family Name : Aranaeidae

Size : 3 to 30 mm

Weight : Female can outweigh male by up to 1000 times

Web diameter : Up to 1 m

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Spiders occur worldwide and show a high degree of diversity. Currently there are an estimated 42 751 species of spider that are known scientifically, but many spiders have yet to be identified to the species level and 400 to 500 species are added yearly. Spiders fall under the Class Arachnida of the Invertebrata. Arachne is the Greek word for spider which in turn is derived from the Greek-Roman myth of Arachne who was a great weaver, but not a goddess, who boasted that her skill was greater than that of Athena, the goddess of wisdom and strategy. Athena did not like this boast and set up a weaving contest between the two of them. However, the  superb tapestry that Arachne wove so annoyed Athena that she destroyed the tapestry and loom, slashed the face of Arachne and ultimately turned her into a spider.

The class Arachnida is morphologically diverse as it includes the spiders, scorpions, false scorpions, whip scorpions, ticks, mites, daddy longlegs (harvestmen) and others. They fall in the Phylum Arthropoda which includes all the invertebrates with segmented abdomens with or without appendages, are oviparous and do not undergo metamorphosis amongst others. Fossilized spiders are known from the Cambrian to the Cretaceous Periods that occurred from 543 to 138 million years ago. The most primitive type of spider is known as a trilobite and it lived in the oceans some 390 million years ago. It often appears as fossils in old sedimentary deposits. The trilobites had no backbone and a hard shell supported the body. There was a head, thorax and tail (three lobs = Trilobata), was 30 mm wide and they became extinct some 250 million years ago.

The goliath bird-eater spider Theraphosa blondi of the rain forest regions of South America is the largest spider known and is a fawn- to blond-coloured tarantula. Its leg-span is only second to that of the huntsman spiders (Palystes) that are widely spread in the temperate to tropical parts of the world but are not dangerous to humans. The goliath bird-eater spider, however, has the heaviest mass. Its common name was gained when it was seen eating a hummingbird by Victorian explorers. It uses deep burrows underground in wetlands and are hairless on their bodies, abdomens and legs. These spiders can have a leg-span of 30 cm and weigh over 170 g. However, their venom is fairly harmless. Despite their name they mainly eat insects and other invertebrates but will also eat rodents, birds, bats ad venomous snakes.

The most dangerous spider in the world is the wandering spider Phoneutria nigriventer of Brazil as only 0.006 mg of its toxin is enough to kill a mouse. In other parts of the world the widow spiders of the genus Latrodectus are highly poisonous with the small brown one being most poisonous but the larger black one, which is 50 per cent as poisonous as the brown one, injecting more poison per bite because it inflicts deeper bites. In Australia the funnel-web and red-back spiders are members of the same group.

In Africa most spiders are generally small and harmless but there are some aggressive, venomous, massive and scary ones too. In the savannas of South Africa some 1499 types of spider occur, of which some 308 are endemic because they occur nowhere else. The 40 species of large baboon spider are tarantulas and are not venomous but can inflict a nasty bite which causes dizziness, vomiting and a weakness in humans. They can become up to 7.6 cm long and live for 25 years.

The 60 species of button spider in Africa are types of widow spider and are venomous with the venom being a neurotoxin. Their colour varies from cream to black and only some have prominent red markings on the body. The bite from a black button spider can kill a child and seriously affect an adult. Sac spiders, however, bite more humans than any other type of spider because they often occur in homes. They have distinctive long front legs that they waive around when hunting. The common house species is generally pale green, tan or straw-coloured. The venom contains a cytotoxin which kills cells and breaks down tissue. Sac spiders are less than 2.5 cm in length and in the field they are usually brownish to red. The small violin spider, of which there are 15 species, occurs widely but it is highly venomous and the cytotoxic venom causes blistering sores. Another dangerous spider of Africa is the six-eyed spider that can kill a rabbit, but no human fatalities have been recorded yet.

www.leopard.tvThe orbweb spiders belong to the family Aranaeidae which contains over 4000 species and 156 genera in addition to many undescribed species. They inhabit a wide range of habitats and include the common orbweb garden spider. Orbweb spiders often have a flat carapace, heart-shaped or triangular sternum, eight eyes in two rows, strong and vertical chelicerae (mouth parts), legs with three claws and numerous spines, variably coloured abdomens, two book lungs and a body size of 3 to 30 mm. Extreme sexual dimorphism occurs with minute males and huge females. Those that are active by day are more brightly coloured than those that are active by night.

The family Aranaeinae of the orbweb spiders is the only subfamily where the male has a palpal (elongated segment) in the femur without a baso-central tubercle (a small, rounded protuberance). In South Africa there is only the genus Argiope with the yellow orbweb spider Argiope cuspidata the probable species. It is also one of the banded garden spiders. The largest orbweb spider in Africa is the rare golden orbweb spider Nephila komaci that occur in the sand forests of Tembe in Tongaland of north-eastern KwaZulu-Natal. Its body is some 3.8 cm long and has a leg-span of 10 to 12 cm.

In South Africa the orbweb spiders of the family Aranaeidae includes 40 genera of master weavers. They are diverse, have various body shapes, colours and camouflage systems, and are harmless to humans. Some are active by day and others by night and those that are active by day use the same web for several days. Eventually the web dries out and becomes ineffective to capture prey. In contrast those that are active by night will spin a new web every night and take it down and eat it at dawn because it is rich in protein. However, the main bridge line of the web is retained for re-use. The webs are usually spun between bushes and therefore orbweb spiders do not compete with other types of spider of prey. Once prey is trapped in the web it is approached cautiously and is wrapped in silk to immobilize it. It is then moved to the centre of the web where it is killed and eaten. Enzymes are pumped into it from the mouth parts of the spider through the fang holes to liquefy the tissue to a soup which the spider drinks.

The genus Argiope includes large and spectacular orbweb spiders that often have a strikingly coloured abdomen and occur in temperate to warm climates. These spiders are commonly known as garden spiders. The large, circular web has a zigzag centre that is known as the stabilimentum because it helps to stabilize the web. Most of the web is practically invisible but the stabilimentum makes it more visible. It is made of white, silky strands and reflects ultra-violet light that attracts prey to the web. The bottom of the web is high enough above the ground to prevent smaller vertebrate animals from breaking it when they pass underneath it. When an orbweb spider is being threatened by a predator it will drop off the web, retreat to the edge of it, or rapidly pump the web in bursts of up to 30 seconds.www.leopard.tv

The males of the orbweb spiders are much smaller than the females who can outweigh a male by up to 1000 times. A male is attracted to the female by pheromones which she releases but he remains waiting at the edge of the web until she has caught a prey. As soon as the female has wrapped the prey up in silk, brought it to the centre of the web and killed it to start feeding, the male approaches her and mates with her. When the female stops feeding and prepares to discard the prey remains he retreats to the edge of the web again to wait for another prey to be captured when mating is repeated. This routine can last for several days. After mating, the female lays her eggs and place them in a silk sac that is then secured to the web. Such a sac can contain 400 to 1400 eggs and is composed of multiple layers of silk that protect the eggs from damage although some insects do parasitize them. The young spiders emerge from the sac in the spring.

Argiope is one of the most common genera of orbweb spider. The carapace of one that was recently seen at Shayamanzi is silver with yellow and black bands on the abdomen and banded legs. An orbweb spider sits head-down in the web with its feet in pairs in the shape of a cross and with the front pair resting on the stabilimentum. The web can be 1 m in diameter. Like most spiders, the orbweb spiders are harmless to humans and they eat insects which is beneficial to gardeners. They can catch and eat prey animals that are almost twice their size. They might bite when they are being handled but their venom, is not medically serious for humans. To the contrary, the venom contains a whole series of polyamine toxins that are potentially therapeutic medically.

 

References:

Dippenaar-Schoeman, A S and R Jocqué 1997. African spiders: an identification manual. Handbook 9, Pretoria: Plant Protection Research Institute.

Larsen, N. undated. Family Aranaeidae (orb-web spiders). Biodiversity Explore. Cape Town: Iziko Museums.

Newlands, G 1988. Spinnekoppe en skerpioene van suider-Afrika. Cape Town: Struik.

Wikipedia 2012. Argiope (spiders). http://en.wikipedia.org.wiki/Argiope_(spider)

Wikipedia 2012. Goliath bird eaters. http://en.wikipedia.org.wiki/Goliath_birdeater

Zijima A undated. Dangerous spiders in Africa. Africa Travel. http://goafrica.about.com/od/africanwildlife/tp/Africas-Scariest-Spiders

article by: Prof J du P Bothma

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