Aricia artaxerxes (allous G.-Hb.) ssp. vandalica Kaaber and Høegh-Guldberg flies for such a long period, almost 3 months, that it might be regarded as two generations, just as is the case with the sibling species, A. agestis Schiff. Experiments with simultaneous rearing of the two species, under identical conditions, exhibited a great difference (I) in the rate of growth of the larvae and (2) in the dispersion in time of the larval stage. On examination of previous experiments these findings were confirmed and must be considered as permanent specific differences. The consequence is that eggs of artaxerxes from a single experimental day of oviposition may result in imaginal emergence spread over a long period (3-4 weeks); in nature the dispersion will be so much greater that the long flying time can be explained without assumption of the presence of two broods. The larvae of agestis grow more equally and rapidly, enabling this species to produce 2 (or 3) generations.