The relations between 5 species of Papilionidae (Papilio machaon, P. hospiton, P. alexanor, Zerynthia rumina, Z. polyxena) and 3 species of Pieridae (Anthocaris euphenoides, Euchloe crameri, E. tagis) from southeastern France and their foodplants are described and analysed. The level of alimentary specificity ranges from strict monophagy to chemically selective stenophagy. Under a coevolutionary perspective, it is not possible to bring to light specific defences of the plants against their phytophagous enemies. On the contrary, close adaptations are evident in the latter, particularly with respect to phenology. Species with a wide geographical range display a low foodplant specifity and are multi-brooded, while local species are monophagous or nearly so and single-brooded ; mixed strategies may be observed. Egg-laying and foodplant research female behaviour must be adjusted as well as possible to the exigences of larval stages. However, the precise coadaptations which are implied in this process are often imperfect The dependence of the phytophagous insect towards its host is two-sided : the host provides not only food but defence compounds and shelter. There is sometimes an adjustment between size and the amount of food available. Monophagous species (except Zerynthia) display a marked tendency to self-regulation of their density, while those with a broader spectrum appear rather more limited by predation, parasitism and diseases. Interspecific competition is revealed in certain species by a «character displacement» phenomenon which insures niche partition in the regions where the competing species live together. In other cases, mullerian mimicry is observed in caterpillars of species using the same foodplant.