As compared to water loss from eggs of several other species, eggs of range caterpillar,Hemileuca oliviaeCockerell, are quite resistant to desiccation. Range caterpillar eggs lack aeropyles, and gas exchange apparently occurs through the micropyle. Loss of water from eggs is stimulated by exposure to low vapor densities or to high temperatures. Temperature stimulates embryonic development, and vapor densities as low as 7.6 g/m3stimulate eclosion.A higher proportion of variation in water loss occurs among egg masses than within masses. Egg size contributes to this variation, with small eggs losing water at a significantly higher rate than large eggs. Weight of larvae eclosing from eggs is inversely proportional to water loss, but larval survival does not seem to be affected by water loss. Eclosion was obtained from eggs losing 33 to 57% of their water.