Answer: It sounds like those are the tunnels of the leaf miner. This insect lays an egg in the leaf which hatches into a larvae that tunnels between the upper and lower leaf surface. After a time, it eats through the leaf and drops to the ground to pupate and emerge as an adult to repeat the cycle. You'll notice the tracks start small and get larger as they go. The growing larvae is at the big end!
Leaf miners usually do not warrant control. Their feeding does little damage to a plant. It is also difficult to control them as the larvae are protected inside the leaf. Keep your plants healthy and growing, and they'll replace lost leaf area with new leaves.
They have natural enemies which seem to keep them from becoming epidemic. Some gardeners with just a few plants will check their plants as they work in the garden and squish the larvae (remember they're at the big end of the tunnel) with their finger and thumbnail.
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