Phenotypic Variation in Australian Eurema Species

Abstract

Australian butterflies in the tropical pierid genus Eurema (the grass yellows) exhibit seasonal phenotypic changes in both size and colour pattern. In four of the five species, smaller adults were present during the summer and autumn wet season than during the cooler winter-spring dry season. The fifth species, E. hecabe, showed the reverse pattern, being largest when it first appeared late in the wet season and becoming smaller as the dry season progressed. When reared in the laboratory, however, all five species produced larger adults when reared at lower temperatures, and at least two of the species also produced larger adults at shorter photoperiods. In the case of E. hecabe, juvenile overcrowding and resource limitation may prevent the potential size increase during the cooler dry season from occurring. All five species had darker markings during the cooler dry season, and in two of them, E. herla and E. laeta, there was also a change in the ground colour of the underwing from yellow to pink or brown. The darker markings may serve a thermoregulatory function, as in other pierids, but the change in colour probably functions to allow E. herla and E. laeta, which diapause as adults during the dry season, to be more cryptic as the grasslands they occupy dry off and change colour. In all species except E. herla, the range of variation in dark markings observed in the field could be reproduced in the laboratory by varying the temperatures and/or photoperiods at which the juvenile stages were reared (darker individuals were produced at lower temperatures and shorter photoperiods). The change in ground colour in E. herla and E. laeta, however, was not reproduced in laboratory rearings, either by changing photoperiod and temperatures, or by rearing animals at low humidities, suggesting that an additional cue may be needed to induce this shift in phenotype.