Almost every plant in my garden has at least one or two leaves like this.
On the upper side of the leaf there are huge amounts of very wiggly white / pale yellow lines. Sort of as though something is burrowing inside the leaf. On the underneath you can see the lines, and also an occasional flat 'egg sac' type thing.
I have found a very small insect (2mm big) in one of them, on another leaf I found some white moth type things (2mm big). There are also greenfly and black fly on some leaves, but I presume its not them leaving the lines?
Oh, and now you've read so far, and identified them , what do I do about them?
OK, I've found the beasties. Nasty, horrible, leaf sucking creatures . I don't think I can take so many leaves off each plant, but I'm starting to scrape them off individually. Are there any good websites for recognising bugs?
Here's a bunch of pictures of leafminers in North America but if you google leafnminer pictures I expect you could get pictures of some additional bugs. Also you could look at pictures of leafminer leaf damage and compare with yours to see if that's what this is.