This is a female specimen. The heavy zig-zag portion of the web is known as stabilimenta. Its function is unknown, but it is thought to resemble writing, hence the common name. Only diurnal spiders use stabilimenta. More at: http://www.cirrusimage.com/spider_argiope_trifasciata.htm
The animal was identified by Nelson Ferretti, La Plata, Argentina. This female was collected during the day, from her tunnel under a medium-sized rock.