Experiments on the Demography of Tropical Butterflies. I. Survival Rate and Density in Two Species of Parides

Abstract

Estimations of number of individuals and survival rate for Parides anchises and P. neophilus have been made in an area in the Northern Range, Trinidad, West Indies, where the insects are particularly common. The change in numbers observed reflects the onset of the wet season, during which the species become most abundant. The estimated survival rate is 0.84 per day, indicating an expectation of life of between 5 and 10 days. This is much shorter than the possible duration of life known from observation of specimens in insectaries, and is similar to the values obtained for a variety of Lepidoptera in temperate regions. It is suggested that one factor contributing to low survival is the very heavy rain showers that were experienced. NEOTROPICAL BUTTERFLIES ARE noted for the striking mimetic associations found among them. Although mimicry is of great significance in the experimental study of evolution, there is as yet little information available on the density, longevity, or dispersal behaviour of the species involved. Such information is essential for a full understanding of the processes giving rise to mimicry. The present observations concern the ecology of two miillerian mimics, Parides anchises (Linnaeus) and P. neophilus (Hiibner), studied in the Northern Range of Trinidad. These butterflies occur in clearings and along paths in the lower montane rain forest of the Northern Range and in the secondary scrub on the lowland. As a rule they are solitary in behaviour, and although during July and August one or two individuals would almost certainly be encountered during a few hours collecting along the traces in the forest, it is not easy to predict a locality for them with any precision. The estimation was carried out in an area where they were more than usually common. Being distasteful and robust insects, we expected them to exhibit a high survival rate in this natural colony.