Leaf Miners. Generally pose no threat to established plants.

Dave

Here is a link that might be useful: Leaf Miners

Yep, that's it. Thanks, and what a relief that my tomatoes aren't in danger.

If you look close at the end of the trail, you can see the criter. If there is a hole in the leaf at the end of the trail, the critter has already pupated and "left the nest".