by Lenny Vincent
The Preservation News. October, 2000
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.