Every growth, marking, bump, or blemish on a plant was made by something, and surprisingly often the cause can be closely traced to a particular animal. I could see from a distance that these hickory leaves had orangish spots on their underside.
On close examination the spots were furry balls! These little growths are galls that have grown around insect eggs, in a weird bit of mostly harmless and stunningly common parasitization.
These orange tribbles hide and protect the larvae of the hickory gall midge (Caryomyia sp.). The creature inside is a helpless pinpoint of a maggot that will grow into a fly so small that it would otherwise go completely unnoticed by humans.