Note: Diptera larvae may live in a corridor mine, a corridor-blotch mine, or a blotch mine, but never in a case, a rolled or folded leaf, a tentiform mine or sandwiched between two more or less circular leaf sections in later instars. Pupation never in a cocoon. All mining Diptera larvae are leg-less maggots without a head capsule (see examples). They never have thoracic or abdominal legs. They do not have chewing mouthparts, although they do have a characteristic cephalo-pharyngeal skeleton (see examples), usually visible internally through the body wall. The larvae lie on their sides within the mine and use their pick-like mouthparts to feed on plant tissue. In some corridor miners frass may lie in two rows on alternate sides of the mine. In order to vacate the mine the fully grown larva cuts an exit slit, which is usually semi-circular (see Liriomyza huidobrensis video). The pupa is formed within the hardened last larval skin or puparium and as a result sheaths enclosing head appendages, wings and legs are not visible externally (see examples).

1a > Leaf-miner: Mine linear, whitish, both upper and lower surface. Pupation internal, at the end of the mine with the anterior spiracles projecting through the epidermis (Spencer, 1976: 433). Upper-surface, less often lower-surface corridor. Frass in isolated grains. Pupation within the mine, in a, usually lower-surface, pupal chamber (Bladmineerders van Europa). A long whitish upper surface corridor, which eventually goes lower surface (British leafminers).

Two highly polyphagous species of Chromatomyia, with indistinguishable mines, have been recorded in Britain. These are syngenesiae (Hardy) and horticola (Goureau). British records of horticola and syngenesiae on Asteraceae hosts not based on examination of the genitalia of reared males are treated here as Chromatomyia 'atricornis'.

Chromatomyia 'atricornis' has been recorded on Tripleurospermum in Britain.

Chromatomyia horticola (Goureau, 1851) [Diptera: Agromyzidae].
OR
Chromatomyia syngenesiae Hardy, 1849 [Diptera: Agromyzidae].

1b > Leaf-miner: A narrow linear mine, even in the finest subdivisions of the leaves (Spencer, 1972: 77, as matricariae; Spencer, 1976: 478). Very fine corridor, upper- or lower-surface, even in the narrowest leaf segments. The corridor may be up to 14 cm long (Sehgal, 1971a). Generally the corridor descends towards the leaf base. Frass in pearl chains of loose grains, hardly in strings. Pupation outside the mine (Bladmineerders van Europa). Pupal posterior spiracles each with 18-20 bulbs.

On Achillea millefolium, Anacyclus pyrethrum, Anthemis, Tanacetum vulgare, Tripleurospermum, Tripleurospermum maritimum and Tripleurospermum maritimum x inodora in Britain and other Asteraceae elsewhere. Widespread in Britain and continental Europe. Also recorded in Canada.

Phytomyza pullula Zetterstedt, 1848 [Diptera: Agromyzidae].