This here is what they call a leaf mine. Essentially an insect gets inside of the leaf where it feeds; the excavation leaves a distinct mark or pattern as you see here. If you’re interested in this sort of thing — i.e. the little clues and bits of evidence of their presence that bugs leave behind — Charley Eiseman wrote the (North American) book. Track and Sign of Insects and Other Invertebrates: A Guide to North American Species, which I highly recommend. He also has a great blog, Bug Tracks, that is well worth a look. (I also like his blog’s tagline.)
On the photo side of things, I spent a good amount of post-processing time on this one, but I’m not sure if the results were worth it in the end. I reduced some of the reflected light, but it left the shot flat in sections, as far as the colors go. I also tried to make up for some color differences that seemed to be caused by the flash with some success. A lot of times these things are subtle fixes, more noticeable at the higher, original resolution, which is where I try to get basal elements correct before making more destination-specific versions, e.g. a this one for the web.