Cecidomyiidae, spatula - Neolasioptera impatientifolia
100 Acre Wood, Winneshiek County, Iowa, USA
July 14, 2011
Size: 2 mm
From another unsuccessful attempt at rearing jewel weed, Impatiens pallida gall flies.
Gagne's The Plant-feeding Gall Midges of North America, page 22: "the third instar (rarely the second also) has a spatula, which is a sclerotized, usually elongate, dermal structure on the venter of the prothorax."