Life History Traits in Different Life Forms of Predaceous Stenus Beetles (Coleoptera, Staphylinidae), Living in Waterside Environments

Abstract

This study was performed to comprehend the extent of life history variation in three predaceous Stenus beetles, showing different habitat preferences in their littoral environments such as moist plant debris (S. juno), bare grounds (S. comma), or wet reed vegetation (S. pubescens). Dissection of field-collected females and counting of laid eggs in laboratory rearings revealed opposing reproductive strategies in the three investigated species: S. pubescens laid few but large eggs, whereas S. juno and S. comma produced many small eggs. These differences are discussed with respect to the predictions of current life history models. Under laboratory conditions, reproductive effort was twice as high in S. juno as in either of the other two species. Since no considerable differences in food consumption could be established, this is considered the consequence of additional reproductive constraints having occurred in the specific habitats of S. comma and S. pubescens, respectively. In S. comma, increased metabolic rates and locomotory activities demonstrate that foraging on open sites is costly in terms of total expenditure in these beetles. These costs can quantify the difference in reproductive effort with respect to S. juno. In S. pubescens, low metabolic rates, concurrent with moderate reproductive efforts, are interpreted in terms of energy minimizing because of a diminished food supply facing this plant-mounting species.