A Very Stable Stabilimentum

My favorite find this summer was a veritable crowd of Argiope in the meadow behind my house.

I have spent countless hours in these fields, but had never noticed the Common Garden Spider, Argiope aurantia. Before I ascertained its harmless identity, I had a few moments of cold terror as I began spotting up their webs all around; at least 20!

This distinct fellow is on page 68 of our lab book Spiders and Their Kin, along with some of its genera.

This particular post was prompted by Thursday's lesson, particularly the bit about stabilimenta, which A. aurantia proudly displays. As we heard in lecture, the functional hypotheses for these lovely decorations are (1) predator defense and (2) prey attraction. 

Stabilimentum visible vertically aligned with A. aurantia's body.

(personal photo)

In my cursory research, results indeed seem to be inconclusive. Some studies note that when starved, stabilimentum production seemed to decrease in Argiope species (Seah and Li, 2002 and Blackledge, 1998). They thus argue it an unlikely prey attraction technique, since it decreases when spiders are presumably seeking bugs--despite low energy output.  Tso 1998 disagrees, as their study finds that the presence of stabilimenta in webs of Cyclosa conica increased prey capture more than any other factor (web diameter, etc). It remains to be seen if there is conclusive variation by species or habitat, or perhaps a continuum of explanations. 

Where we can all agree, I think, is the artistry of the endless variations:

(source: Seah and Li, 2002)

As a side note, I also have an Argiope mystery on my hands.

In my enthusiasm for those in my backyard, I started to see A. aurantia & kin everywhere! A pondside hosted what I thought were Argiope bruennichi, cleverly suspended between cattails. 

I didn't get a shot of the ones I saw, so here's a stock photo...

(Source: Wikimedia Commons)

Sadly, page 68 of Spiders and Their Kin thinks A. bruennichi are only in Europe, and the widely renowned Wikipedia seems to agree, though I'd've bet my collembola collection I was right. Does anyone have light to cast on what I may have seen? Have they hopped the pond?