I found several Hawthorn leaves with these small spiky looking balls on their surface. When I looked closer, I saw that some had been opened and larvae were sticking their heads out. Here you can see one unopened gall on the right, two with larvae emerging, and two where the larvae may have left. I don't know which species of insect made these galls, but possibly Dasineura crataegibedeguar, a gall midge.