Check out leaf miners - various insects that lay their eggs on the foliage and then the eggs/larvae burrow into the leaf to feed on the succulent interior tissue. Leaf miners tend to be more cosmetic in their damage than causing actual harm to the plants.

Just realized I forgot the second picture

That second image looks like the damage is done on the reverse of the leaf... I bet you can see them if you peek nice and quiet. Could be several pests.

You might consider spraying a diluted Neem solution, which basically makes the plant 'taste icky' without killing beneficial insects. As a beekeeper, I really hope you don't choose an insecticide that ends up killing any ol' bug just for pretty leaves. Lots of dahlia growers do, though.

I don't like to use insecticides, although I have used slug bait. Doesn't seem to be helping. I try to use natural stuff, though.

U can use Diatomaceous Earth, it's natural but make sure to sprinkle underneath the leaves & at base of plant also. ( or use a duster)