Egg Weight Variation and Lack of Correlation between Egg Weight and Offspring Fitness in the Wall Brown Butterfly Lasiommata megera

Abstract

When bred in the laboratory, and given 30% sugar solution as food ad lib., females of the wall brown, Lasiommata megera, laid eggs that decreased significantly in weight over the oviposition period. All of the eggs laid by 13 females were weighed individually, and eggs varied in weight from 0.33 to 0.65 mg. If females had laid small eggs only they could have increased the number of eggs laid by 25%, from 182 to 228, while keeping the reproductive effort unchanged (i.e. the total egg mass laid by the females). To test if heavy eggs, or larvae derived from heavy eggs, were in any way superior to light eggs, or larvae derived from light eggs, the following fitness variables were tested in relation to egg weight: 1) egg mortality at high, 260C, or low, 14'C, temperatures, 2) egg mortality at low, 30%, 45%, relative humidities compared to that at 100% RH, 3) larval development time, 4) pupal weight attained, 5) larval survival when given no access to food for 0, 1, 2, 3 or 4 d as newly hatched larvae, and 6) ability of newly hatched larvae to establish themselves on a coarse grass, Festuca rubra, compared with their ability to establish themselves on a soft grass, Poa annua. No positive correlation between egg weight and any of the above measured fitness variables was found. Although it can be argued that the evolution of unbridled fecundity in butterflies is counteracted by the fact that there is an inverse relationship between the number of eggs laid and the time allocated to the female's finding a suitable site for each egg, this does not explain why butterfly females do not lay small eggs only. The apparent lack of correlation between egg size and offspring fitness indicates that the decrease in egg size over the oviposition period may be best explained as a non-adaptive trait.