25 September 2011

Argiope Spiders

Late summer/early fall seems to be the season of spiders.  This time of year, it is difficult to walk through the woods, fields, or wetlands without wearing webs on your face and pants.  Some of the most common and conspicuous spiders that I notice this time of year are those in the genus Argiope.  Often called "garden spiders," these ~2" long (the bodies of the males are much smaller) yellow, black, and silvery white orb-weavers can be quite intimidating for those with even the slightest bit of arachnophobia, but unless you have six legs and a pair of antennae, you have nothing to fear.  A severly harassed female with an egg sac may bite a human, but the bite is said to be no worse than a bee sting.

Female Black-and-yellow Argiope.  Be sure to click on this photo to enlarge it so that you can see the characteristic zig-zag stabilimentum that she weaves into her web.

An interesting feature of the argiope spiders is that they weave a ribbon of silk called a stabilimentum into their webs; this stabilimentum is often in a zig-zag pattern.  Although entomologists cannot agree on the true purpose of the stabilimentum, there are several theories as to why argiope spiders expend energy to create them.  One possibility is that it adds to the stability of the web.  Another idea is that the stabilimentum is easily seen by birds and mammals that as a result avoid the web and therefore don't destroy it.  Others think that the structure helps to camouflage the spider, while some think that the stabilimentum may actually attract potential prey.  Regardless of its purpose, each species of argiope spider produces a distinctive stabilimentum that helps biologists distinguish between species by only looking at the webs.

There are five species of Argiope in North America, with a sixth (Bruennich's Argiope, Argiope bruennichi) potentially present in Alaska.  Of the five species in the continental United States, three have fairly restricted ranges.  The Silver Argiope (Argiope argentata) is primarily known from the southern parts of California, Texas, and Florida, as well as from Arizona.  A similar species, Argiope blanda, is known in the United States only from the southern tip of Texas.  Florida Argiope (Argiope florida) can be found in the southeastern United States, from North Carolina to Florida and west to Louisiana; it may also be found in Arizona.  The other two species are widespread.  The one I see most frequently is the Black-and-yellow Argiope (Argiope aurantia).

Female Black-and-yellow Argiope, dorsal side.
 Black-and-yellow Argiopes have a characteristic black and yellow pattern on the back of their abdomen, whereas the carapace is covered in silvery hairs.  The legs are mostly black but often have reddish or orangish coloration at the proximal end.  Black-and-yellow Argiopes also have distinctive yellow and black coloration on their undersides.  You can find this species of spider in old fields, wet meadows, riparian areas, and gardens.

Female Black-and-yellow Argiope, ventral side.
 The other widespread species is the Banded Argiope (Argiope trifasciata).

Female Banded Argiope, dorsal side.
 Banded Argiopes are identified by the yellow, black, and silvery-white stripes on the back of their abdomens.  Like the previous species, the carapace of this species is covered with silvery hairs.  The legs of the Banded Argiope are striped black and orangish-brown.  The underside of the abdomen of the Banded Argiope is also distinctively patterned with black and yellow.  You can find the Banded Argiope in grassy and shrubby areas that are usually a bit drier than where you find the Black-and-yellow Argiope, but the two can be found in the same area.

Female Banded Argiope, ventral side.
 One of my observational differences in the females of these two species is that the Black-and-yellow Argiope seems to drop from her web fairly quickly when I bump into it, whereas the Banded Argiope seems to hold her ground.

If you are one of those people who is deathly afraid of spiders, I hope that this at least gives you a better respect for the argiopes.  If you are one who likes spiders, be sure to get out in the next couple of weeks before it gets too cold to admire these large black and yellow arachnids.


Anonymous said...

Great photos!

Question - I only ever see the Black-and-Yellow in August and September. Any idea what happens to them the rest of the year?? They are far too distinctive and large to just overlook.


Scott Namestnik said...

Thanks Cath.

Black-and-yellow Argiopes only live one season. The eggs hatch in the spring, and the spiderlings continue to mature throughout the year, with the females (the conspicous ones) reaching maturity in late summer. When immature, their coloration is more cryptic, and they are well camouflaged in their stabilimentum. I guess that's whey we don't notice them until later summer.

Beth said...

One of these spiders built a web on the front door (the glass one) of the house where I grew up. I could open the inside door and watch the spider grab insects and repair her web. I don't want them ON me, but they're fascinating to watch. I noticed the zigzag pattern at the center of the web, and always just thought it was to strengthen and stabilize the center. Thanks for the interesting information!

Scott Namestnik said...

You're welcome, Beth! I bet your spider was fun to watch. I've seen then wrap up grasshoppers... pretty entertaining.

Katt said...

Hi, Scott!
Thank you for taking the time to post this information about this spider; sadly, last year, we killed a large one of these spiders off our deck (I thought it looked vicious) and then this year, we have four on our front porch in the open....one of my neighbors said she knew they were poisonous (so I looked at your posting and learned not to worry) Thank you...I hate to kill anything that is beneficial to our well-being (destruction of mosquitoes) -thank you again (and I;m sure if the spider could type she'd say thank you!)

Scott Namestnik said...

Hi Katt. Thanks for your message, and glad I could help save a couple spiders this year. Nothing to worry about with argiopes!

Ruby Fox said...

Hey I had one yesterday and was planning to watch her Cycles with the babies but before she could have the babies a birds swooped up and got her I was disappointed too because I've seen these things in my backyard when I was 10 years old I'm not 15 and know that throwing a stick at them so they'd go away was a mistake I really hope to see the babies in Spring I'm really disappointed do you think there will be another female coming to my yard

Scott Namestnik said...

Probably Ruby... they're not uncommon.