The Facebook page for SpiderIdentification.org is really busy these days. It is no surprise. Spiders in the northern hemisphere, especially orb weavers, are reaching maturity now. Larger spiders spin larger, more conspicuous webs, often in situations where people notice them more often. The chief attention-getter in the United States right now is the Black and Yellow Argiope, Argiope aurantia.
The females are very large spiders, with a body length of 19-28 mm. Their bright Rorschach pattern of black and yellow might set off your “dangerous arachnid” radar, but no orb weaver of any kind is considered by scientists to be dangerously venomous to people or pets. Males, by contrast, are tiny, only 5-9 mm in body length, and magnitudes smaller in terms of body weight. Females need to build up energy reserves to be able to produce eggs.
The female Black and Yellow Argiope spins a rather small orb web given her size, usually in tall grass or shrubs no more than two or three feet off the ground, and usually lower. There, her snare can intercept large insects like grasshoppers.
A distinctive signature in the webs of most Argiope species (there are at five species north of Mexico) is a thick, zigzag band of silk running down the center of the web. This structure is called a “stabilimentum,” and its function remains something of a mystery. It may serve to shield the young spiders, which confine the stabilimentum to the hub of the web, from harm. The young spider quickly zips to the other side of the web when it feels threatened. Another hypothesis is that the stabilimentum is like a beacon on a tall building: it advertises the presence of the web to birds in flight so that the avian animal won’t destroy the web by accident. This comes at a cost, however. The presence of a stabilimentum can reduce prey-catching success by as much as thirty percent (Blackledge and Wenzel, 1999). That statistic also flies in the face of yet another hypothesis: that the stabilimentum most likely functions as a lure. The silk band stands out bodly in the ultraviolet end of the light spectrum, and many insects seeking flower nectar may mistake it for a raceme of flowers. Not all individual spiders spin a stabilimentum, and one might assume that webs spun higher in the vertical plane would be more likely to have one if the purpose was to deter bird strikes.
Look for male spiders lurking on the outskirts of a female’s web. They may be attracted to the female by a pheromone she emits (Olive, 1982). Approaching cautiously, a male may eventually be able to couple with the gargantuan object of his affection. He inserts one of his pedipalps into one side of the female’s epigynum (female external genitalia), and quickly pumps his sperm into her, hopefully before she renders him a meal. He will repeat the process with his other pedipalp on the opposite side of the female’s epigynum, if he is able. Should he succeed even once, he usually breaks off the tip of the pedipalp, which remains stuck in the epigynum. This “mating plug” therefore prevents any competing male from inserting into that opening (Foellmer, 2008).
Once mated, a female produces one or more egg sacs, each about the size of a large marble, and covered in tough, papery silk. Inside are 300-1,400 eggs. The eggs hatch in late autumn or early winter, but the spiderlings do not exit the egg sac. Instead, they go into diapauses, a dormant state with lowered metabolism. They emerge the following spring and reach adulthood by late summer. Various parasites and predators can take their toll on the egg sacs and spiderlings, however. One study found that 19 species of insects and 11 species of other spiders emerging from the egg sacs of Argiope aurantia. Chief among the parasites were the ichneumon wasp Tromatopia rufopectus, and the chloropid fly, Pseudogaurax signatus. The overwhelming predators of the egg sacs are birds. Nearly every egg sac found in the wild during the study had suffered damage from birds (Lockley and Young, 1993).The Black and Yellow Argiope is also known as the “Writing Spider,” named perhaps for the stabilimentum, Golden Orb-weaver, and Yellow Garden Spider, among other aliases. I grew up learning it as the “Golden Garden Spider.” The species ranges from coast to coast in the United States, but is absent from arid regions.
Enjoy your encounters with these remarkable spiders while you can. The first frost is likely to claim their lives, if they do not perish in some other way before that date. See if you can determine what kinds of insects they are preying on, and whether their web location changes. There remains much to learn about even our most common and conspicuous arthropod neighbors.
Sources: Blackledge, Todd A. and John W. Wenzel. 1999. “Do Stabilimenta in Orb Webs Attract Prey or Defend Spiders?” Behavioral Ecology. Vol. 10, No. 4, pp. 372-376.
Foellmer, Matthias W. 2008. “Broken genitals function as mating plugs and affect sex ratios in the orb-web spider Argiope aurantia.” Evolutionary Ecology Research. Vol. 10, pp. 449-4462.
Lockley, T. C. and O. P. Young. 1993. “Survivability of Overwintering Argiope aurantia Egg Cases with an Annotated List of Associated Arthropods.” Journal of Arachnology. Vol. 21, No. 1, pp. 50-54.
Olive, Cader W. 1982. “Sex Pheromones in Two Orbweaving Spiders: An Experimental Field Study.” Journal of Arachnology. Vol. 10, No. 3, pp. 241-245.