Answer: You're describing classic signs of leafminer damage. Adults are flying insects, and when the female finds a likely spot she makes a tiny slit in the surface of a leaf and lays an egg. The eggs hatch into wormlike larva that burrow between the upper and lower surfaces of a leaf and feed. The road map type lines you see are where the larvae have eaten the inner leaf tissues. Flying insects are difficult to control, the larvae are protected from chemicals as long as they're between the surfaces of the leaves. There are some natural predators that feed on leafminers, so beneficial insects may have already begun to control the leafminer population in your garden.
If you find the damaged leaves unsightly you can pick them off and destroy them. If you squish the leaf, you'll kill the leafminer, which will effectively reduce the population next year.
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