secondary plant compounds
Everyone who sometimes identifies a plant knows how different they are in taste and smell. Each species has its own bouquet. This bouquet doesn’t exist to please us; together with resins, phenolics, latex and countless other chemical substances, collectively known as secondary compounds, their biological functions its to make the plant impalatable, or even downright toxic, to herbivorous insects. Only an insect that in the course of its evolution has specialised on a particular hostplant is capable of breaking through this chemical barrier. Often these specialists has turned around the plant’s weapon: they not only are not damaged by it, they often need these compounds for their development, or as a stimulus to eat.
mono-, oligo-, polyphagous
A leaf miner larva, that spends its entire larval life on a singe foodplant is maximally exposed to the chemical defence of that plant. Most leafminers therefore are restricted to a single plant species, or to a few species that are so closely related (mostly belonging to the same genus) that their bouquets are very similar. These miners are called monophagous. Oligophagous miners, species that live on several, not very closely related plants are less common. And polyphagous species, occurring on a wide range of unrelated plant speciess, are exceptional; an example is Liriomyza strigata.
The strict food specialisation of most miners is a powerfool tool for the identification. But for the identification of the less strict oligophagous species also knowledge of plant classification is necessary. The more closely two plants are related, and the more their bouquets are similar, the easiier it will be for such a miner to make the step van one species to the other. Phytoliriomyza variegata for instance lives mainly on Astragalus (Milk Vetch) and Colutea (Bladder Senna), which is understandable by an inspection of the systematically arranged plant list: they are the only two genera of the subfamily Galegeae. When difficulties are met in using a key, it always is advisable to try the identification in the keys to a few f the most closely related species.