Ecological correlates and heritability of reproductive variation in two co‐occurring grasshopper species

. 1. Relationships are examined between mean egg size and possible selection pressures acting on two grasshopper species, Chorthippus brunneus (Thunb.) and Myrmeleotettix maculatus (Thunb.), co‐occurring at three sites in a sand dunes area. 2. Heavy eggs were laid where the mean crowding of hatchlings was high in relation to the amount of grasses with thin‐edged leaves. To a lesser extent, large eggs were also found in more desiccating conditions. Mean egg dry weights did not correlate with measures of crowding or measures of crowding in relation to total grass abundance. 3. Significant differences in egg size and clutch sizes were observed between the stocks of second generation, laboratory‐reared M.maculatus from three study sites. The significant difference in egg size (site 2 stock > site 1 stock) ran counter to the difference observed in the field but the difference in clutch size (site 3< site 2) was consistent with that observed between field populations. 4. We argue that egg size is primarily determined by selection pressures on the size of hatchlings but that predictably poor feeding conditions for juveniles and adults at site 2 results in the production of smaller adults which, in M.maculatus , then lay smaller eggs. We further argue that M. maculatus from site 2 have been selected to lay larger eggs for their body size and body condition in order to compensate partially for this environmental constraint.