I have seen them before, I call them leaf miners but I'm sure they have a name, they are quite common on various veg, and a nuisance, sorry not very helpful.

I think that they are leaf miners too....even trees and shrubs can be affected

Pick off the affected leaves and burn or destroy somehow. If the whole crop/plant is affected then remove and destroy. That's the 'green' way. If you don't mind chemicals then there are various available that include premethrin but do watch out for spiders and other predators before spraying. Personally I prefer to remove the plant to a remote part of the garden and let nature do its worst to the bugs but this is not guaranteed to stop the infestation. Burning or spraying ... that is the question?!

Thanks to all, they are off to the compost!

What lays them? Just thinking that prevention is better than composting.