The Silver Argiope Spider

Among the many spiders found around the Nature Center, one of the most striking is the Silver Argiope, a conspicuous orb weaver. Seen from the top, a number of lobes are apparent along the sides of the broad abdomen of the female. On older females the front half of the abdomen is silver, while the back half is brown with silver spots. The underside of the spider, hanging upside down in the middle of its almost vertical web, is usually seen first. From this viewpoint, the brown abdomen seems to be crowned with silvery lumps. It is marked by 3 distinct yellow lines which almost form an inverted triangle.

In this area, Silver Argiopes have a preferred habitat, Prickly Pear cactus. In mid- March, the best location around the Nature Center was the large Coastal Prickly Pear below the sand casting on the building's west side. At least 16 older females had webs strung between the cactus pads. A number of younger (these are yellowish-brown) Argiopes hung on small webs which linked adjacent cactus pads. Old, greenish egg sacs lay on the pads or hung from silk strands. (In a life history study carried out in Panama, spiderlings emerged from the egg sacs after 18 to 21 days.) Several discarded spider skins were present. (In the same Panamanian study, the males molted about 6 times before reaching maturity. Mature females are larger than mature males, but take longer to develop, molting about 14 times. )

The Silver Argiope belongs to the spider family Araneidae, the orb weavers. An orb web consists of a central hub (an irregular mesh) from which a number of spokes radiate. Immediately outside the hub, in a region called the attachment zone, is a narrow spiral used to adjust the position of the spokes. The thread used for the hub, the attachment zone spiral and the spokes is not sticky. Sticky thread is used to form a spiral reaching in from the outer part of the web, but stopping before the attachment zone. The small gap between the two spirals is called the free zone. The web is tilted slightly, apparently to aid movement across the web.

Sometimes, older Silver Argiopes decorate their vertical webs with one to four zig-zag diagonal bands called a linear stabilimentum. Such bands start near the ends of the spider's outstretched legs (in or near the attachment zone) and stop at or before the sticky thread of the spiral zone. The spider aligns its legs with any linear stabilimentum, so that, if all four bands are present, the center of the web appears to be marked with a large X. Of the 16 older females mentioned earlier, ten made no stabilimenta, 5 made portions of one band in the lower half of the web, and one made 3 diagonal bands. Instead of constructing linear features, young Silver Argiopes sometimes reinforce the hub region substantially. This thickened structure is called a disc (because of its shape) stabilimentum. (These are easier to see if one looks across the hub rather than through it.)

The significance of the stabilimenta is not known. I will discuss the work that has been done on these next month. In the meantime, observe the Silver Argiopes beside the building and apply your experience and interpretive skills to see what explanations for the stabilimenta you can deduce. If you are unsure of your identification, the Silver Argiope is shown in the Audubon Guide to North American Insects and Spiders. The Black-and-yellow Argiope or Golden Orb Weaver also shown is present in San Diego County but I have not seen it around the Nature Center. It sometimes constructs a vertical stabilimentum. Several other species of spider coexist with the Silver Argiopes in the cactus. The only other ones with orb webs (in mid-March) were the much smaller Cyclosa spiders. These spiders often construct vertical stabilimenta which can remain long after the rest of the web has been destroyed. (Look for the Cyclosa female in the center of its web.)

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Ron Lyons (volunteer 1990-1999)
Chula Vista Nature Center, 1000 Gunpowder Point Drive, Chula Vista, CA 91910-1201