St Andrew's Cross Spider
Argiope keyserlingi is found in eastern Australia, in habitats ranging from rainforest margins to open forest and heathland (QLD, NSW, VIC).Features:
St Andrew's Cross Spiders are named for their bright web decorations - zig-zag ribbons of bluish-white silk that form a full or partial cross through the centre of the orb web. The spider sits with its legs in pairs. In females, the carapace is silvery and the abdomen is banded with silver, yellow, red and black above and two longitudinal yellow stripes below. The brown and cream coloured males (body length 3 to 4 mm) are smaller than females (body length 10 to 16 mm).Ecology/Way of Life:
These spiders build small to medium-sized orb webs, occupied day and night, on low shrubby vegetation. Their prey includes flies, moths, butterflies, bugs and bees. Larger more dangerous prey are usually secured by silk wrapping into a neat parcel before being bitten - smaller prey may be bitten before wrapping - and then eaten.
The purpose of the cross-like web decoration, called the stabilimentum or cross, has long been a puzzle. At first, many, it was thought that it it strengtheneded or "stabilised" the web, but more recently biologists believe that it helps to capture prey or avoid predators. The ribbon-like silk of the stabilimentum reflects ultra-violet light strongly. Such light is attractive to flying insects, which use it to find sources of food, such as flowers, and to navigate through openings in the vegetation. If the cross attracts insects it may increase the web's insect efficiency in catching prey.
The stabilimentum may also make the web and its owner more obvious to predators active during the day, such as birds and wasps. However, it varies a lot in shape, from a complete cross to a partial cross with from 1 to 3 arms, or may be absent altogether. This variation may confuse predators searching for the typical full cross. The cross may also warn predators like birds to stay away - the effort needed to clean off the sticky silk after diving through the web may put off predators from trying again.
The cream-coloured young spiders make a circular stabilimentum (like a white silk doily) that disguises them well and may also act as a sunshade. As the spider grows the "doily" is gradually transformed into a cross.
When threatened, the spider either drops from the web or shakes it so vigorously that both spider and stabilimentum become a blur, again probably confusing its attacker. This doesn't always succeed, as indicated by empty, damaged webs and the presence of these spiders as food in the mud cells of wasps.
Mating occurs in late summer and autumn and can be dangerous for the small males. One or more males sit in the upper parts of the web - some may be missing legs, survivors of encounters with unreceptive females. The male constructs a mating thread within the web, onto which he attracts a receptive female by vibrating the thread. The female suspends her pear-shaped egg sac in a network of threads, - often among leaves where it is disguised by the sac's greenish silk. Despite this, the egg sacs are often the target of parasitic wasps and flies.
Spiderlings disperse by ballooning in spring but some juveniles can already be seen in webs in autumn.Interaction with Humans/Threats:
No threats have been identified for these spiders.Other Comments:
Named by Friedrich Karsch in 1878; the genus name is from the Greek Argiope = a nymph or graceful young woman; the species name honours Dr E. Keyserling, the 19th century student of Australian spiders.Further Reading:
Craig, L.C. and Bernard, G.D. (1990). Insect attraction to ultraviolet-reflecting spider webs and web decorations. Ecology 71 (2): 616-623.
Eisner, T. and Nowicki, S. (1983). Spider web protection through visual advertisement: role of the stabilimentum. Science 219: 185-187.
Elgar, M.A. (1991). Sexual cannabilism, size dimorphism and courtship behaviour in orb-weaving spiders (Araneidae). Evolution 45 (2): 444-448.
Elgar, M.A., Allen, R.A. and Evans, T.A. (1996). Foraging strategies in orb-spinning spiders: Ambient light and silk decorations in Argiope aetherea Walckenaer (Araneae: Araneoidea). Australian Journal of Ecology 21: 464-467.
Main, B.Y. (1973). Spiders. Australian Naturalists Library. Collins, Sydney.
McKeown, K. (1963). Australian Spiders. Angus and Robertson, Sydney.Topics: Spider webs Sexual cannabilism Silk Prey capture
Text & map by Mike Gray; photograph by Mike Gray, courtesy of the Australian Museum.Sponsorship welcomed:
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