The larvae of the four main groups of leaf miners -Diptera, Hymenoptera, Lepidoptera, and Coleoptera- generally are not difficult to tell apart
Mining fly larvae always are maggots: more or less spindle-shaped, feet and eyeless beings without an apparant head. As a matter of fact, the head is strongly reduced and withdrawn into the thorax. Two pairs of spiracula are invariably present. See the discussion in Diptera for more details.
In the three groups that remain, after the Diptera have been eliminated, the chitinised head always is apparent, and, in most cases, thoracic feet.
Agromyza nigripes on Flote-grass
Mining sawfly larvae always have thoracic feet, although they may have been reduced to mere stumps. Moreover, in most species up to six pairs of prolegs (abdominal feet) are present; they may have undergone a strong reduction however. Often there is neck-like constriction between head and pronotum. The eye consists of but one ocellus.
Fenusella wuestneii on Osier
Moth larvae may have some resemblance to sawfly larvae. The eye usually consists of more than one ocellus. The head often is more or less retracted into the pronotum. Thoracal legs and prolegs generally are present, but may be reduced or absent. Prolegs, when present, may be simple stumps like in sawflies, but more often they have an arc or circle of tiny crochets at their tip. Moth larvae never have more than 5 pairs of prolegs.
Epermenia chaerophyllella on Hogweed
Beetle larvae generally are squat. In the most species-rich families, Apionidae and Curculionidae, thoracic feet are absent. Leaf beetle larvae (Chrysomelidae) do have feet; they are yellow or greyish, broad, dorso-ventrally flattened animals. The larvae of Buprestidae are strikingly black-and-grey with relatively slender feet.
solitary or communal
A fairly reliable character in relation to larvae is wether they are solitary or communal. Of course, in solitary species it may happen that several mines on the same leaf coalesce. An inspection of the start of the min(s) and their frass pattern generally is sufficient to deduce what happened. Also the possible presence of a group of empty egg shells will give an indication.
late or sluggish larvae
When a larva sits in the mine with little or no movement, it mostly is parasitised, even though this is not always visible witout dissecting it. The same applies to larvae that, compared to their conspecifics, are unusually late to pupate or leave the mine.