Aceria macrochela is a gall mite that causes solitary galls to form on the leaves of Field Maple. The galls are seen more often than the mite that causes them. The gall is rounded, 2 to 4 mm across, with hard walls often in angle between veins; inner surface with many-celled hairs.

Aceria myriadeum (aceriscampestris) also causes galls to form on Field Maple, but the galls are smaller and not confined to vein axils - but indeterminate examples may be found 

Gall Adult

Only on Field Maple.  State host plant and provide a photo

On the leaves of Field Maple.

Galls visible in summer.

Frequent and widespread in Britain.

Fairly frequent in Leicestershire and Rutland.