Key for the identification of the known mines of British
Diptera recorded on Mentha

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).

See Key to non-Diptera.

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, usually in a lower-surface puparial chamber.

A long whitish upper surface corridor, which eventually goes lower surface.

Two highly polyphagous species of Chromatomyia, with indistinguishable mines, have been recorded in Britain. These are syngenesiae (Hardy) and horticola (Goureau) which can only be distinguished by the male genitalia. Both species are widespread in Britain and elsewhere, although syngenesiae is almost entirely restricted to Asteraceae. Records on Asteraceae not based on examination of male genitalia are treated in this account as Chromatomyia 'atricornis'.

Chromatomyia horticola is recorded on 160 plant genera in 31 families, of which 55 plant genera in 19 families, including Mentha, in Britain.

Chromatomyia horticola (Goureau, 1851) [Diptera: Agromyzidae].

1b > Leaf-miner: Mine beginning with a small spiral, later developing into a greenish blotch, frequently at leaf margin, brown when old. Pupation internal or external (Spencer, 1972b: 89 (figs 296), 90).

Upper surface. The mine begins as a short, compact brown spiral, followed by a secondary blotch, often at the leaf margin. Often spiral and blotch are separated by a recognisable corridor segment. Secondary feeding lines conspicuous (in fresh mines). Pupation outside the mine; sometimes pupation occurs earlier but then the exit slit already has been made, and the spiracula do not penetrate the plant epidermis.

The mine starts with a small spiral and then develops into a greenish blotch (frequently on the leaf margin) which turns brown as it ages (British Leafminers).

On Mentha aquatica, Mentha longifolia and Mentha suaveolens in Britain and additional species of Mentha elsewhere. Widespread in Britain. Also recorded in the Republic of Ireland. Widespread in continental Europe.

Phytomyza tetrasticha Hendel, 1927 [Diptera: Agromyzidae].

1c > Leaf-miner: Mine beginning with a small spiral, later developing into a greenish blotch, brown when old. Pupation frequently in mine (Spencer, 1972b: 90).

The mine, that may be upper- or lower-surface, generally starts with a tight spiral. Its loops are so tight that the leaf tissue dies off and turns red; nevertheless the corridor remains well visible. Later the corridor widens and finally a large secondary blotch develops around the initial spiral. Frass at first in two rows of grains, later in pearl chains. Feeding lines very clear. Pupation now within, then outside the mine.

On Clinopodium, Mentha and Nepeta in Britain and Clinopodium and Satureja elsewhere. Doubtfully recorded from Oxford in Britain. Widespread in continental Europe.

Phytomyza obscura Hendel, 1920 [Diptera: Agromyzidae].

1d > Leaf-miner: An irregularly linear mine which may become a secondary blotch. Pupation normally external, but not infrequently the puparium remains in the leaf at the prepared exit slit (Spencer, 1972b: 86 (fig. 289), 88; Spencer, 1976: 467 (fig. 822, 468).

Upper-surface corridor, often branched and crossing itself, but not forming a secondary blotch. Frass mainly in thread fragments. Pupaption generally outside the mine. Sometimes pupation occurs within the mine, near the lower epidermis, but then already an exit slit has been made, and the spiracula do not penetrate the epidermis (Hering, 1924a, 1957a).

On Mentha in Britain and elsewhere. Probably not uncommon in Britain. Widespread in continental Europe.

Phytomyza petoei Hering, 1924 [Diptera: Agromyzidae].

1c > Leaf-miner: A linear-blotch mine, first instar mine is linear, later developing into a conspicuous white blotch. Frass greenish diffused (Spencer, 1972b: 46, 47 (fig. 138); Spencer, 1976: 166).

The mine begins with a long, upper-surface, slender corridor. After a moult the larva changes its behaviour, and makes a large, upper-surface primary blotch without apparent feeding lines. Often the blotch overruns more or less the initial corridor. Frass in the corridor liquified to form a wide green band, with a few tiny black granules along the sides. Pupation outside the mine.

A narrow gallery leading to a largish blotch on the upper surface. Frass is green and indistinct in the gallery - small grains may be seen at the gallery edge.

On numerous genera of Lamiaceae, but not yet on Mentha, in Britain and elsewhere. Widespread and locally common in south of England. Also recorded in the Republic of Ireland and widespread and frequently common in continental Europe.

Amauromyza labiatarum (Hendel, 1920) [Diptera: Agromyzidae].