Gregarious, social caterpillars have stimulated research because group size may affect survival, growth rate, thermoregulation, and interactions with other species, yet group size is often variable both within and among populations. We used a combination of observations and experiments to study the importance of group size for egg and larval survival in the Glanville fritillary butterfly, Melitaea cinxia, which lives in groups from egg hatching until the last larval instar. Both experimental manipulation of egg clutches placed in the field and observations of naturally occurring groups showed that survival increased with increasing group size. This pattern was present independently during all four developmental stages studied: eggs, prediapause larvae, diapausing larvae and post-diapause larvae. However, it was significant only during two stages: pre-diapause (in one year only) and diapausing larvae (in all years). Large group size increased survival of entire larval groups as well as that of individual larvae within surviving groups. These results may explain why cluster size is large and why adults oviposit infrequently. Large cluster size, coupled with correlated survival of group members, in turn helps to explain the unstable local dynamics and short average persistence time of local M. cinxia populations.