Critter Corner
The Preservation News. October, 2000
© 2000

by Lenny Vincent

Garden Spiders of the Coastal Sage Scrub

Three species of the large and colorful garden spiders live in our coastal area: Argiope aurantia, the black and yellow garden spider (AKA, writing spider); Argiope trifasciata, the banded garden spider; and the silver backed garden spider, Argiope argentata. The last two are commonly found in coastal sage scrub.

All Argiope are daytime sit and wait predators that hang head down in the center (hub) of their large and conspicuous orb webs. To avoid attracting predators such as birds or wasps, these spiders remain motionless unless they are handling prey caught by their sticky web. Activities like egg sac construction, web repair, and defecation take place off the web and at night.

Prey that is safe to handle is usually bitten first than wrapped in silk. The first pair of legs rotate the prey, pairs two and three hold to the web, and the last pair is used to cast silk pulled form the spinnerets over the prey. Dangerous prey such as wasps are usually wrapped in silk first then bitten.

A. trifasciata adult females have an oval abdomen that is marked on top by alternating transverse silvery white or yellowish broad bands and narrow dark bands. The top (dorsal) surface of the cephalothorax has silver hairs. The legs have dark spots on a yellow background. Members of the genus Argiope show extreme sexual dimorphism with females being much larger than males; trifasciata is no exception. Females are 15 to 25 mm long and the adult males, which are seldom seen unless they are in the female's web preparing to mate, are about 1/4 the size.

Adult female A. argentata have a wider abdomen relative to its length than does A. trifasciata. Additionally, the abdomen has three pairs of lateral lobes. The front half of the abdomen is yellowish while the back half has a triangular pattern that is dark with a few white marks. Adult females are 12 to 16 mm in length, males are 3.7 to 4.7 mm long. Again, it is easiest to find adult A. argentata males when they are in the female's web.

You can often find heavy zig-zag bands of silk known as stabilimentum in the free zone of the webs of Argiope. The free zone is just outside the hub and contains only radial threads. Stabilimenta of A. argentata, take the shape of the letter X. Stabilimenta of A. trifasciata appear as an incomplete X. Sometimes the stabilimenta of Argiope is incomplete or even missing.

The function of the stabilimentum has been debated for a long time. It is no longer thought to stabilize the web. It may warn birds from blundering into the web, it may hide the spider from predators, and since it reflects ultra violet light, it may attract prey.

Lenny Vincent teaches General Biology and the Biology of Insects and Spiders at Fullerton College, and is on the Board of Directors of the Laguna Greenbelt, Inc.