The illustration above is courtesy of Dover Publications. I believe these are sphinx moths and their larvae, one of which is shown munching on a leaf. Caterpillars eat leaves in order to survive and grow up to their adult forms. Without insect damage, no caterpillers, no butterflies and moths.
Right now I can look out the window at a vast expanse of snow, which arrived far earlier this year than it ordinarily does. But on the Internet, something is always in bloom! In my files I found this link to an August entry from one of my favorite online newsletters, "The Week at Hilton Pond." Published by the Hilton Pond Center for Piedmont Natural History, based in York, S.C., these weekly updates never fail to be informative, and are usually illustrated with beautiful photos. I particularly enjoyed this unconventional look at insect-damaged leaves, in which the usual ("Quick, get the insecticide!") response is replaced by a naturalist's appreciation for natural beauty wherever it's found. As you look at these photos, keep in mind that plants are able to withstand substantial insect damage without suffering any ill effects. Typically if a plant dies as a result of insect activity, it was already weak for some other reason. So except for extreme infestations, it's rarely necessary to run for the insecticide just because you see a chewed leaf. It's quite ok to just stop and enjoy what Hilton Pond calls "the artsy side of insect damage."