THE SIGNIFICANCE OF EGG SIZE VARIATION IN BUTTERFLIES IN RELATION TO HOSTPLANT QUALITY

Females of three tropical satyrine butterflies in the genus Mycalesis lay variable-sized eggs. The species also differ substantially in their mean egg weights with M. perseus laying smaller eggs than M. terminus and M. sirius. The adaptive significance of the egg size variation was examined in relation to three offspring fitness components: larval survival, larval developmental time and pupal weight, to establish if larvae derived from heavier eggs were in any way better than those derived from lighter eggs when reared on different hostplant qualities varying in leaf toughness and nitrogen content. When the three species were reared on the tougher nitrogen-poor host quality (Panicum-intermediate category) larvae from heavier eggs had significantly better chance of surviving and reaching adulthood than those from lighter eggs. However, when larvae were reared on the softer nitrogen-rich host quality (Panicum-soft category) the advantage to heavier eggs diminished in M. terminus and M. sirius, but not in M. perseus. Egg weight was also positively correlated with subsequent pupal weight in M. perseus for both male and female larvae reared on Panicum-soft, and with larval development rate in M. sirius (both sexes) when reared on Themeda - a hostplant known to be of poor quality for M. sirius. The positive correlation between egg size and offspring fitness suggests that under field conditions larvae from larger eggs may do better than those from smaller eggs when conditions are unfavourable, for example, adverse climate or poor host quality. Moreover, a strong positive correlation between egg size and the width of the head capsule of newly emerged first instar larvae suggests that larvae from larger eggs have bigger mandibles and therefore potentially able to chew and survive better on tougher foliage. Potential selective pressures likely to influence egg size in these satyrines and other butterflies are discussed.