As a first detailed approach towards the fine structure of the eggshell in members of the family Arctiidae (Hexapoda: Lepidoptera), deposited eggs ofUtetheisa ornatrixfrom the subfamily Arctiinae are described. Scanning electron microscopy revealed that the eggs are dorso-ventrally depressed spheres and attached with the flattened basal pole to the surface of the food plant. At the upper pole, cross- or star-shaped micropyles occur at the center of a small zone characterized by high reticulation consisting of straight, polygonally arranged elevations of the chorion. The flanks are not reticulated and possess aeropyles in their upper portions, but are smooth in their lower part. The architecture of the chorion was studied using transmission electron microscopy. It is ca. 4 μm thick in the upper flank, but decreases to ca. 1.1 μm at the base. The values are at the lower end of the spectrum found in the Lepidoptera. The chorion has an elaborate substructure in the upper flank, but assumes a homogenous texture towards the base. The entire surface is covered by mucous secretions that are very prominent around the transition from the flanks to the bottom. A highly sculptured and thick chorion may be interpreted as an effective protective device for the developing embryo. The eggs ofU. ornatrixare smooth over wide areas and possess a thin chorion. This may mean that the eggshell plays only a minor role in the protection against predators and parasites. Indeed, the work of others has shown that the eggs ofU. ornatrixare protected through large amounts of pyrrolizidine alkaloids that they receive from their mothers. We assume that with this chemical defense in place, an elaborate eggshell can be dispensed with in the moth.