During the past three years of observation, I've found that the Banded Argiope (A. trifasciata) are usually the earlier species to appear in the garden. They tend not to grow quite so large as A. aurantia. That said, both species grow very quickly and are conspicuous by their size. Although they will have been growing for quite some time before their presence becomes obvious in the garden, they are still comparatively small when first noticed in their circular webs which are referred to as "orbs". Their bodies are slim and flat and look small in relation to their legs. Their colouring tends to be less brilliant -- usually more silvery than the brilliant yellow with black markings seen a little later in the season. At this stage, the spiders are generally quite shy and will drop from their webs into the surrounding foliage when approached. The above spider was photographed on August 27, 2005. The same spider was photographed again on September 13th. Notice the difference in body shape as well as markings. The abdomen becomes very enlarged shortly before the spider departs from its web to create an egg case in a suitable location.
Based on my observations, the bulk of the diet of these big spiders seems to consist of grasshoppers. I believe it's no real coincidence that these spiders do the bulk of their growing in August and September, which also happens to coincide with the period during which grasshoppers are at their most plentiful and are reaching their greatest size. While other insects are frequently captured in the Argiope webs, it is the grasshoppers that are most frequently seen wrapped in silk when I go about checking the webs several times a day.
Many of us have vivid memories of encounters with these large orb-weaving spiders. Our first encounter may have occurred while running through a field and suddenly finding our way blocked by an immense web with a huge spider hanging at the center. It is not due to the exaggeration of memory and time that we recall these webs as having been 2 or 3 feet across. In actual fact, the web of a mature Argiope is easily that size and its silk very tough and resilient. Most webs have a zigzagging "stabilimentum" running across the web (see above photo), generally vertically, although there can be much variation in its shape, size and direction. It is believed that the stabilimentum may help to attract insects to the web, or conceal the presence of the spider which often sits at its center.
The above photo is of the underside of an Banded Argiope. Note the "spinnerets" at the top end of the abdomen closest to the rear of the spider. They release the silk which is used for spinning webs, wrapping captured insects, and for producing an egg case. An interesting side note to this photo, and something that I'll be watching for in the coming season, is that it seems that these spiders spend the brightest and hottest part of the day on the side of their webs away from the sun, which also means that their underside would be exposed to the greatest amount of heat during the day. I noticed this on several sunny days when the weather was unusually hot.