Information on general morphology of flies and midges (Diptera) is given by McAlpine, 1981, Hennig, 1973.

Head
The major terms are explained in the picture Li trifolii head.pct.
The Agromyzidae belong to the highly diverse dipteran group Cyclorrhapha that is well characterized by the presence of the ptilinum just above the antennae. It is a sac-like organ which is inflated at the beginning of the adult life to facilitate tearing the puparium in order to escape from it. The following figures of heads of Liriomyza trifolii show a partly everted ptilinum Li trifolii ptilinum.pct and a head with fully retracted ptilinum Li trifolii frons.pct.
The surface of the ptilinum can be covered with scales or denticles in a variety of sizes.
The knowledge of this feature can be useful to understand the life history of some Agromyzids (see Subgenus Poemyza).
In matured adults only the ptilinal fissure ( head structure.pct, Li trifolii frons.pct) is visible, marking the area, where the ptilinum had been everted. This area is called lunule ( head structure.pct).

The frons can be subdivided into two structural different areas: The frontal vitta above the ptilinal fissure is weakly sclerotized, bristles and hairs are lacking. In mining flies, heavier sclerotization in the frontal area is restricted to the ocellar triangle, the frontorbital plates (frontorbits) next to the eyes and the dorsal part of the head called the vertex. These parts have often an other color and surface structure than the frontal vitta.

The size, direction and presence of some bristles and hairs located on these sclerites might be of diagnostic importance. They are all illustrated on this picture:
head bristles.pct
Frontorbital bristles - Their number usually ranges from 3-5 on each side. The lower frontorbital bristles are normally more or less inwards directed (medioclinate). The number of clearly medioclinate bristles can be employed for identification. (In the literature they are often referred to as "ori" and "ors".)
Frontorbital setulae - These setulae are minute, sometimes not easy to find. They can be either erect, directed to the occiput (reclinate) or to the antennae (proclinate) (see orbitalsetulae id.pct). In some cases they can be missing.
Vertical bristles - The bristles themselves are constant in position and appearance but the color of the cuticle surrounding their base is frequently used as diagnostic feature, especially in species groups with extensive yellow coloration.
Ocellar bristles - highly constant, not used for agromyzid taxonomy.
Postocellar bristles (postvertical bristles) - also highly constant.

The compound eyes are usually oval shaped, often with an angular hind corner. In some Cerodontha, Tropicomyia and Japanagromyza the eyes can be more circular and larger. If the head surface is mainly yellow, narrow dark frame often surround the eye margins.
The area below the eyes is called gena or cheek ( head structure.pct). It may differ somewhat in size or color pattern. In Japanagromyza and some Ophiomyiini the genae are conspicuously small. The lower margin of the genae is often darkened. In the literature is often distinguished between 'cheeks', a membranous area immediately below the eyes, and 'jowls' the membranous area below the cheeks up to the head margin Spencer, 1976 b. This difference is avoided here since a clear separation is often impossible as also admitted by Spencer.
Anteroventrally at the corner of the genae (vibrissal corner) a single pair of medioclinate bristles called the vibrissae is present in all agromyzid flies head bristles.pct. In some male Ophiomyia the area can be conspicuously enlarged and broadened. In that case it is called vibrissal fasciculus Ophiomyia head.pct.

Forming a proboscis with a wide apical labellum the mouthparts belong to the nonbiting, lapping and sucking type widespread among higher Diptera. The flies are able to ingest liquid food and liquefy dry substances e. g. dried honeydew. As diagnostic characters the mouthparts have hardly been used except for Ophiomyia pinguis where the proboscis is extremely enlarged ( Oph pinguis head.pct).

Antennae head structure.pct
As in other cyclorrhaphan flies the apical segments are highly modified.
The third antennal segment (first flagellomere) is enlarged. Inserted subapically the remaining flagellomeres are reduced to a slender, bristle-like arista Ps atra antenna.pct.
The normally rounded but sometimes angular shape of the third antennal segment can be of some taxonomical importance.

Thorax
The major terms are explained in the pictures thorax dorsal.pct (bristles) and thorax lateral.pct.
The humeri are mostly blackish, only rarely they show yellow or whitish coloration. In lateral view, the anepisternum (mesopleuron) and the katepisternum can be diagnostic through the variation in color-patterns. Furthermore, the scutellum can be either blackish or conspicuously yellow. Sometimes the surrounding of the scutellum can be yellowish as well. The mesonotum is mostly dark, either covered with pubescence or shining.
The setae on humeri, anepisternum and katepisternum are either generally rather constant throughout the whole family or in detail variable within species. Therefore they are only rarely used as diagnostic characters.
There is more variation in the bristles on the mesonotum and scutellum, see Figure ( thorax dorsal.pct). Some of the bristles and hairs shown in the drawing can be absent; especially the dorsocentral bristles at the presutural part of the mesonotum are missing in Ophiomyiini, Japanagromyza and Pseudonapomyza. Furthermore the prescutellar acrostichal setulae (or simply "prescutellar bristles") can be enlarged or as small as the neighbouring acrostichal setulae. One pair of the scutellar bristles is missing in the subgenus Cerodontha. The density of acrostichal setulae (or simply "acrostichals") can vary considerably among species.

The legs are of limited significance in agromyzid taxonomy. They show some color variation, sometimes even within species. On the fore and middle tibia sometimes one or more outstanding median spinules can be distinguished from the small hairs the legs are covered with. Those species show a stridulation mechanism (von Tschirnhaus, 1972, see below) and bear a stridulatory scraper on the hind femora. This is a sharp long edge situated at the inner side.

Except in a few species in tropical rain forests (Tschirnhaus, 1991) the wings are hyaline and translucent, never infuscated. Some conspicuous tracheae that are called "veins" run through each wing. The position of the wing veins and the spaces embraced by veins, which are called "cells", are of fundamental value for identification of especially higher taxa. For example the two agromyzid subfamilies are distinguished by the shape of the wing veins subcosta and the first radial vein. The critical area is shown here: Oph pinguis wing2.pct. The next picture ( agr-phy-wing.pct) illustrates different shapes in Agromyzidae.

In the figure wing.pct a wing with a complete set of veins and their nomenclature is shown. It is a traditional convention that primary veins are abbreviated with capital letters whereas cross veins and cells, which are often named after the neighbouring primary veins, are expressed by small letters.

The most widespread modifications are: Reduction or basal shift of the two cross veins (mainly Phytomyza and related taxa); reduction of the length of the costa and "costalization". That means the further anterior shift of the most anterior veins next to the costa wing Pseudonapomyza.pct, wing N lateralis.pct.

Near the wing base the following structures might be of some significance for identification of agromyzids:
Tegula: Sclerite at the base of the costa with lateral hairs of varying color Oph pinguis wing1.pct.
Squama thorax lateral.pct, Oph pinguis wing1.pct : Basal lobe of the wing, the color of the margin and the marginal hairs can be important for species identification.

The halteres ( thorax lateral.pct) can be interpreted as atrophied second pair of wings although they have got an important sensual function. Operating as pendula during flight, they serve as sensual organ. By the position of the oscillating halteres, the fly can detect and control wanted and unwanted rotation movements of the body. The coloration of the halteres (white or at least partly darkened) can be of importance for identification of agromyzid subgroups.

Abdomen
The abdomen consists of 5 pregenital segments in male and 6 in female. The first one is rather short and closely associated with the second tergite. It is only recognizable through an incomplete suture (adventitious suture).

The abdominal tergites are usually dark, brown to black, sometimes with metallic coloration. Both the joints and the margins of the tergites can be yellowish or white. Several species have got some yellow patches at the lateral sides of the tergites.
As diagnostic character this coloration can be misleading because it often depends on the condition of the specimen. The joints between the tergites are usually better visible in pregnant females and in specimens stored in alcohol. In dry specimens the joints are normally inflated and therefore concealed.

The tergites are covered with fine hairs (setulae) of different size. At the posterior margins the hairs are often longer than elsewhere. Especially in females the posterior hairs of the last two tergites can be conspicuously elongated.
Stridulation mechanisms in Agromyzidae were discovered by von Tschirnhaus, 1972 consisting of a file at the first abdominal segment (e.g. Li trifolii strid.pct) and a scraper at the hind femora ( Li trifolii tibia.pct). The stridulatory file is a field of scales or small spines that produce sound when it the scraper rubs it. Although the sound itself is still not unambiguously confirmed there is little doubt about that function because similar structures have been observed in other insects. The stridulation mechanism evolved at least two times in Agromyzidae. It is present in Agromyza, most Liriomyza and some Cerodontha species. In Agromyza the stridulation file can be found in both sexes at the lateral margins of the first tergite ( Ag albipennis strid file.pct Ag intermittens strid file.pct). The stridulation organ of Liriomyza and Cerodontha shows some structural differences to that of Agromyza indicating an independent evolutionary history: It occurs only in males and the stridulation file is situated on the connecting membrane between tergite and sternite (picture series with successive increased magnification: Li trifolii strid.pct, enlarged: Li trifolii strid2.pct, Li trifolii strid3.pct). The relationships between the very similar stridulation mechanisms of Liriomyza and Cerodontha are not yet fully understood.