Now is a great time to look at Hart's Tongue Fern for feeding signs of the micro Psychoides filicivora. The larva feeds on the spore cases of the Hart's tongue Fern which at this time of year are rusty brown. Feeding signs are typically raised areas on the spore cases that look like the stuffing is being pulled out of a cushion. A picture of the larva is below, note the brown head and prothoracic plate. The closely related P. verhuella is also found in Somerset; the larva of that species has a blackish head, prothoracic plate and anal plate.
Psychoides filicivora larva