A small spider web under the midday sun diffracts light in a most impressive way.
This effect is emphasized by the shallow DoF produced by the 100mm macro F2.8 L IS USM wide open.
Basically, this is just a very special bokeh.
Please note that the part of the web in the background produced some very interesting geometrical shapes. At any rate, this image is placed under the sign of geometry; the amazing work of the spider should not be hidden behind the optical miracle.
The full resolution picture contains almost one million different colours (934517 to be precise), which I find rather impressive for a simple web!