Drawing heavily upon natural history data from the Neotropical butterfly fauna, an attempt is made to develop a model, with testable hypotheses, to account for the evolution of egg-clustering and larval gregariousness. Given the high diversity of both plant and butterfly species in the American tropics, there is a higher incidence of egg-clustering there, including some species with aposematically-colored immature stages. Emphasis is placed on the need to examine both the physical (mechanical) toughness of larval food plants for larval feeding (...) success, and the spatial structuring of food plant populations. It is argued that when food plant spatial patchiness is high, the breadth of local food plant species usage low, plant growth rates high, fed-upon structures tough, butterfly dispersal low, and reproductive potential low, egg-clustering evolves. Such ecological character states, coupled with the occurrence of food plant patches in large sizes relative to the nutritional and fecundity needs of the butterfly specialist on the species, select for egg-clustering to ensure survival of the butterfly population in that habitat. It is emphasized that the model proposed here requires considerable examination through field studies. (shrink)