Spiders, like other arthropods, have six legs. This blog post concerns a species of spider belonging to the order Araneae, family Araneidae, and genus Argiope (the orb-weaver spiders).
Orb-weaver spiders are the most common group of spiders in the world. Some of them can be bizarre-looking and very brightly colored. I recently found two specimens in my garden. Compared to other orb-web spiders found in southern California, they are unusual. Their bodies (carapaces) have a yellowish-brown color pattern. The exteriors have several rows of prominent bumps that protrude out over the ventral surface.
I checked what literature I have in my natural-history library, as well as online, and I determined that these spiders represent an unusual variety of Argiope argentata, a species that typically lives in New World tropical and temperate regions, especial in of Central America and South America. The specimens in my garden, however, have a less spectacular color pattern than most other Argiope argentata species, which can have patches of silver, black, red, yellow, green, orange and/or brown.
Dorsal view of specimen (A): length of spider approximately 2 inches long). Next, below, is the ventral view (B) of this same specimen (slightly out of focus). The prominent white, “zig-zag thread” = the stabilmentum, which was secreted by the spider. See the text for a discussion of the function of the stablimentum. [Note: this kind of spider positions itself upside down on its web. The wind was blowing when I took this picture, and I could not get a sharp-focus image of ventral (bottom side) of this spider].
The last and third image (C), see below, is a ventral view of another, but smaller, specimen (length only1.25 inches). Its stabilmentum is also present (on the left side of this view). The web of this specimen was more complete than the other specimen. Both specimens were found in my garden, in northern Los Angeles County, southern California.
Orb spiders typically have very large and complex orbicular (circular) webs. The web of the first spider photographed above (images A and B), was only a very partial one (i.e., a small lenticular-shaped remnant of its recently damaged web). I returned the next day to view the spider, again and it and the web were gone, except for a few filaments. Orb-weaver spiders are known to routinely destroy their own webs and create new ones. That is most likely the best explanation for the sudden disappearance of its web. The web of the second specimen was a more complete (but still small) classic orb-spider web.
Like other orb-web spiders, the ones found in my garden positioned themselves upside down in their web where they wait for their prey (e.g., mosquitoes) to get trapped.
note: a comment on the purpose of the stabilmentum. Some experts speculate that it is to make the web visible to birds that might otherwise fly into the web and, thereby, destroy it. This type of structure is commonly made by orb spiders.
Sources of Information:
Conrad, J. 2014. A blog site. Naturalist Newsletter. https://backyardnature.net.
Levi, H.W. and Levi L.R. 1968. A guide to spiders and their kin. A Golden Nature Guide, New York, 160 pp.
Nicky Bay. Orb-weavers (Araneidae checklist)-Macro photography of many species, some with unbelievable shapes and colors). An online website.
