Known as gall wasps or Gallflies, anthropods in the family of Cynipidae spend a portion of their life, form egg to adult, inside of a gall, and this is what's shown in the spotting. Unfortunately, many other wasps take advantage of the gall wasps' efforts and parasitize their galls by laying eggs within them.
Found smack in the middle of a trail, and the only one I could find. The elevation was about 6,000 feet.
Attached firmly to the leaf.