Butterflies and moths
As an order within the insects Lepidoptera are characterised by their specialised feeding apparatus (but a few primitive famimilies still have biting mouthparts) and scale-covered wings. Metamorphosis is complete. The larvae (caterpillars) have chewing mouthparts and are, almost without exception, vegetarians. There are in principle three pairs of thoracic feet. The abdomen, consisting of ten segments, bears a pair of unarticulated prolegs on segments 3,4,5,6, and 20. (Not always all prolegs are present.) Prolegs have at their tip one or more circles of crochets; their form and arrangement is of diagnostic value.
The large majority of mining Lepidoptera belongs to the most primitive families of the order (Regier ea, 2015a).
The number of stemmata (“ocelli, facets”) in the eye-patch is 6, except for the Eriocraniidae and Nepticulidae, that have but 1. This sometimes is useful to discriminate Lepidoptera larvae from sawfly larvae, that always have but 1 stemma in the eye.
Lepidoptera mines can mostly be distinguished easily from Diptera mines because they are full depth. Fleck mines and tentiform mines are exclusively the work of Lepidoptera. The frass line is broad or narrow, may be coiled; never the frass is distributed in two narrow lines along the side of the wall, like often happens in Diptera mines. Also there are never primary or secondary feeding lines in Lepidoptera mines.
When the mine still contains larvae or pupae the difference is even more clear. Diptera larvae (as far as they are miners) are maggots, without feet and seemingly without a head. Lepidoptera larvae invariably have a more or less strongly chitinised head. Diptera pupae (puparia) have an unmistakable compact saucage-shape.
Perhaps the most important website on various aspects of the European Lepidoptera is Lepiforum.
Kuchlein & Donner (1993a), Regier, Mitter, Kristensen, Davis, van Nieukerken, Rota, Simonsen, Mitter, Kawahara, Yen, Cummings & Zwick (2015a), Stehr (1987a), Sterling, Parsons & Lewington (2012a).