I’m disappearing for the weekend to visit my parents for Father’s Day, but as my sister, whom I’m carpooling with, is late, I have time to share a quick photo.
I found this leaf at Maplewood Bog, one of our three MAPS stations. It was near the edge of a poor fen that curls around the northeastern corner of our site, and I spotted it as I walked the water’s edge looking for dragonflies. Leaf galls aren’t that unusual themselves, but this one was pretty interesting for the pattern – perfectly round, pale spots bordered in dark, vibrant pinks and purples. The leaf itself is a Red Maple, an not uncommon species at the site.
Looking it up in my marvelous Tracks & Sign of Insects by Eiseman and Charney, I learned that these are the work of a Maple Eyespot Gall Midge, Acericecis ocellaris. These tiny midges lay their eggs on the underside of new leaves in May; the larvae form these distinctive circular galls while they feed through June. The galls themselves can be variable in colour, from greenish-yellow to bright cherry red, and may or may not have the colour repeated in a central spot to create the “eyespot” of the common name. Eventually the larvae drop from the leaf to pupate in the soil; once they’re gone, the galls turn a uniform brown as the leaf material dies. The species uses Red Maple almost exclusively as its host plant, with just occasional occurrences of galls on other maples and rare instances on other trees.
seabrookeleckie.com 
thanks for the information. i have often seen these and wondered what they were. galls themselves are pretty interesting, but i know so little about them!
Will it harm the tree.