I like the crazy patterns leaf miners make in the leaves. The traces are made by larvae, mostly of flies or moths, that live between the top and bottom surfaces of the leaf, eating their way around the interior in different ways until they eventually are ready to transform.  By the plant species involved and the pattern left by the larvae you can mostly identify which the species.  I've recorded mines on sugar snap peas, chard and aquilegia; also on weeds such as sow thistles, herb bennett & willowherb.  Mostly they're fairly harmless, though I'm not so happy about the damage to my chard - not that I can do much about it, if I don't want to use insecticide.