Leafminers can be from different insect groups. Many species of Lepidoptera (moths), Diptera (trues flies), Coleoptera (beetles) and Symphyta (sawflies), have larvae which mine plants. It is the larvae of these insects which produce these mines within the leaves of plants, feeding on the plants’ tissues as part of their development cycle. Mines tend to be restricted to a certain range of host plants and so the identification of a miner is facilitated by correctly identifying these plants. The shape of the mines (gallery or blotch) and the patterns of the droppings (frass), besides characteristics of the larvae and pupae, can be diagnostic. It is always best to try and hold the leaf towards the light which makes the frass much clearer.

Lepidoptera – Moths

Oak Carl
Tischeria ekebladella
Horse Chestnut Leaf-miner Cameraria ohridella larvae leaf-mines
Horse Chestnut Leaf-miner
Cameraria ohridella
Nut-tree Pygmy
Stigmella microtheriella
Feathered Bright
Incurvaria masculella
Red Hazel Midget
Phyllonorycter nicellii
Eriocrania sp.
Chestnut Pygmy
Stigmella samiatella
Apple Leaf Miner
Lyonetia clerkella
Elm Midget
Phyllonorycter tristrigella
Common Case-bearer
Coleophora serratella
Base-spotted Pygmy
Stigmella basiguttella
Firethorn Leaf Miner
Phyllonorycter leucographella
Golden Pygmy
Stigmella aurella 
Bordered Carl
Coptotriche marginea
Little Cosmet
Mompha Raschkiella
Willow Pygmy
Stigmella obliquella

Diptera – True Flies

Holly Leaf Miner
Phytomyza ilicis
Liriomyza strigata
Phytoliriomyza melampyga
Agromyza sp.
Phytomyza spinaciae
Aulagromyza sp.

Symphyta – Sawflies

Profenusa pygmaea

Useful resources:

British Leafminers
The Leaf And Stem Mines of British Flies And Other Insects