In search of the elusive Argiope

This morning, Lori Spears, Ben Kuethe and I went to a field located not far from Thatcher, a small town west of Tremonton. Someone had told Lori that there had been large amounts of Argiopes last year in this field in the late summer. Although this genus in the family Araneidae can be quite common in the right type of habitat, I had not seen it yet in the U.S. We followed the instructions to get there, eager to find some big Argiopes.

The field consisted mostly of grasses, thistles, and a few small shrubs:

We walked around for quite a while without seeing any spiders at all, until we came across a couple of large orb webs. The occupants turned out to be Aculepeira spiders, which are in the same family as Argiope. These Aculepeira were big and appeared to be well fed, as suggested by the prey items caught in their webs. The abdomen of Aculepeira is typically egg-shaped, widest anteriorly, and has a dorsal longitudinal lobed band.

Only two or three species of Aculepeira can be found in North America. According to Levi 's publication The orb weaver genera Metepeira, Kaira and Aculepeira north of Mexico (Araneae, Araneidae), the species we found was probably A. packardi, as suggested by the posteriorly directed lobes on the back of the abdomen. I have collected this species at my field sites in Logan Canyon, but I had not seen any as big as the ones here. Adult A. packardi females build a new web every night and actively remove prey caught during the day.

Aculepeira  resting on thistle 

Aculepeira sitting in the hub of her web with a prey item

Aculepeira working on a large prey item

Aculepeira has a typical median ventral white streak, as seen in the picture below:

As we were discussing why we did not find any Argiopes yet, Lori found a small Argiope trifasciata in its web, not very far above the ground! This was the only one we were able to find, but it was a pretty spider that made our trip worthwhile.

Lori finds an Argiope

Argiopes sit on the web with their head facing downwards, and their legs are often held in pairs. Argiope trifasciata is also known as the Banded Garden Spider. The back of its abdomen is a silvery/pale yellow background, with several horizontal black stripes, and the legs are striped. We think our spider was immature, as it was relatively small and had a mostly white dorsal surface.

I have read that Argiope trifasciata prefers to orient its web along an east-to-west axis, with its abdomen facing south. Given that the underside of A. trifasciata is mostly black, the orientation of the spider is supposed to maximize solar radiation and heat gain, which is an important consideration for spiders that are active late in the year.

Argiope trifasciata

The dark underside of A.trifasciata

We decided to return to this site in about a month, in hopes of finding adult Argiopes.