Chirosia grossicauda Strobl, 1899
This small fist-like ball on the end of this fern frond is caused by a fly called Chirosia grossicauda. The larvae tunnel into the central veins of the pinnules in late summer and cause them to roll downwards from the tip. The solitary white maggot feeds on the main vein by mining. Mature larvae most likely pupate in their galls.
Widespread and fairly frequent in Britain.