Accidental discovery of an ultra finestructure of the web of the Garden Cross Spider

Kruisspin / Garden Cross Spider / Araneus diadematus ©GK

Yesterday my attention was drawn by an orange coloured spider illuminated by the sun. I was forced to underexpose the photo by 2 stops to get the right exposure of the spider. It appeared to be the common Garden Cross Spider (Kruisspin). But what surprised me was the thread of the web:

Here is a detail of the thread (divided in two parts for a better view of the details). A remarkable pattern of very fine stripes with all the colours of the rainbow emerged. I've never seen such a pattern. I expected a continuous thread. Why in heaven's name this very detailed structure? What is going on? How does the spider manage to create this pattern? And what is its function (if it has a function at all)?

I was lucky. First, the sun must illuminate the web at the right angle to make the thread and the pattern visible. Spider webs are supposed to be invisible to be effective! In the shadow, you see absolutely nothing. Secondly, I underexposed the photo to get the spider right, and as a side effect the pattern appeared. I almost never underexpose a photo. Thirdly, I was lucky there was no annoying wind that could move the web. Wind makes macrophotography impossible.

The science behind it

I know that scientists have analysed the chemical composition of the threads. The threads are made of Spidroins proteins that form the majority of spider silk fibers. 

Major Ampullate Spidroin Structure. The repetitive domains of major ampullate (dragline) silk consists of alternating regions of polyalanine and glycine-rich sequences. These repetitive domains are important for the strong and elastic features of the dragline silk fibers ( Xu and Lewis,1990). (Synthetic Spider Silk Production)

It seems very unlikely that what we are seeing here are alternating regions of  Alanine and Glycine (Amino Acids) rich regions in the proteins. More likely we see crystalline and non-crystalline regions in the threads? Maybe different spidroins? I have really no idea. There is a lot to explore...