Family Araneidae

I was sitting outside my second home, Rogers Library, when I noticed a very intricate web.  It spread from a low rising bush up between two trees.  Upon further inspection, I saw a spider sitting concentrically.  It was green and spiky.  I had never really seen anything like it.  I was sitting on the bench when the web moved. I wondered if the spider had caught some nummies in its web, so I walked around to get a closer look.

Sorry for the poor quality.  My phone is not as smart as others.  I decided to try and identify this spider, which proved to be a more difficult task than I assumed.  From what I gathered, it is most likely related to the spiny-backed orb weaver which is a species endemic to Alabama.  The orb weaver and the spider pictured above belong to the family Araneidae.  The name orb weaver refers to the round shape of the woven web.

After I took the picture, the spider began to move towards a fly in the upper right region of its web.  And I caught the action!

Sorry for the expletive.  After it finished eating, it swung off the web on a strand of silk towards me.  In the time it took me to refocus the camera on the spider, it had already made its way back to the main web.  I don't think the action was defensive.  I presented no threat, and swinging seems a poor way to defend the web.  I propose that this is a method used to return to the center of the web where it can feel vibrations better.  Swinging to the prey would cause too much movement of the web, and even though the fly was stuck, the spider walks along the web agilely without disturbing the web.  What better way to attack prey than with stealth?  I have never witnessed a spider eat its prey before.  This was a cool thing to happen upon before a nice long study session.

Edit: I went back the next day, and my spider friend was still there.  I looked at her again, and identified her as a spiny-back orb weaver.  I was right on in my assessment, but was hesitant to identify the species with only the picture I had.  I can now confidently say that the spider outside of Rogers Library belongs to the species Gasteracatha cancriformis.  

From a website describing this species of spider: "Females build webs that they use to capture prey. A female sits facing down in her web, awaiting her prey on the central disk. When a small insect flies into the web, she moves toward it, then snaps the radii on either side of the insect. In order to secure the prey, the spider snaps more of the web around the area, then rushes toward the prey in order to determine its exact location and bite it before carrying it back to the central disk where she feeds.

To carry a prey item back to the central disk, the female either climbs back up the web with her food, or swings down a drag line then climbs up to her resting area. If the prey is smaller than the spider, she will just paralyze it, carry it to her spot, and eat it without wrapping it up. If the prey item is larger than the spider, it requires wrapping before being carried to the central disk.Sometimes several prey insects become caught in the web at the same time, so that the spider must find and paralyze them all. If it is not necessary to carry them away to eat them, the spider may just feed on them where they are, then come back to them as she pleases.

Gasteracanta cancriformis feeds upon the liquified insides of her prey. Deliquified carcasses are discarded from the web and are easily recognized in their mummified state.

Foods eaten: drosophilids, whiteflies, beetles, moths, other small fly species (none appear to have been rejected)."

That explains why she swung at me! I was also partially right about it being a way to return to the center of the web.  This is also a good description of what is seen in the video above.