This section is the home of unidentified leaf mines, which may have been made by the larvae of moths, flies, beetles, or sawflies. Knowing the host plant is extremely helpful, and in most cases essential, to identifying the miner.
Great information and images of Illinois moth mines at microleps.org
Mines of British Flies and Other Insects
(not North American, but much of the information is transferable)
(Charley Eiseman's blog posts about leafminers)