Gall formed by two leaves appressed together to form a blister gall...Asphondylia solidaginis Beutenmüller

Each of the two leaves involved forms one half of the unicellular, ovoid, 4–5 mm diameter, white-fungus-lined gall...

Monothalamous. Pale green, rounded somewhat, blister-like, much broader than high, single, or two or three in a row coalescing. Inside it is white, and contains a large larval chamber. It is formed between two, three or four leaves fastened together, the gall protruding on the upper and under side of the leaves. Width, 2.5 to 3.5 mm. Height, 2 mm.

The galls that are formed in spring and early summer are blister leaf galls that join two (and sometimes three or four) leaves together like a snap... When made of two leaves, one leaf contributes the bottom part of the gall and the other contributes the upper part to form a single-chambered gall that is thickly lined by white mycelium on the inside.

[this species is re-described in detail in this paper; please refer to it for many more details of gall biology, insect life history, anatomy, and phylogeny]