When identifying galls, it is crucial to identify the host plant. The host tree is eared willow, so called because of the little ear-shaped "stipules" (appendages at the base of a leaf), three of which can be seen in the right-hand half of the image. For the purposes of gall identification, eared willow (Salix aurita), goat willow (S. caprea), grey willow (S. cinerea), and their various hybrids, are classed as "sallows".

The presence of such trees, which are very common in woodland that surrounds Carman Reservoir, reveals something about the ground conditions there: this is an area of "willow carr", a variety of woodland that arises on very wet soil, especially if it is prone to flooding. In fact, many of the access paths within these woods consist of boardwalks, for that very reason. Willow carr is a valuable habitat that supports many kinds of invertebrates, including the midges responsible for the galls shown in the photo.

As described in "British Plant Galls" (Redfern/Shirley/Bloxham, 2002), these growths are caused by the gall midge species Iteomyia capreae, and are "hard pustules, domed above and conical below, with a circular red-rimmed opening ... on sallows, widespread and common".

The opening on the underside is not very prominent in this side-on view (although it can be seen as a small reddish area at the bottom of the leftmost gall). The overall shape of the gall can be seen to match the description, and the total height (both above and below the leaf) is about 5 mm.