DEFENSE-MECHANISMS OF ARTHROPODS .129. THE CHEMISTRY OF SEXUAL SELECTION

The moth Utetheisa ornatrix (Lepidoptera: Arctiidae) is protected agai nst predation by pyrrolizidine alkaloids that it sequesters as a larva from its foodplants. At mating, the male transfers alkaloid to the fe male with the spermatophore, a gift that the female supplements with a lkaloid of her own and transmits to the eggs. Eggs are protected as a result. The male produces a pheromone, hydroxydanaidal, that he derive s from the alkaloid and emits from a pair of extrusible brushes (corem ata) during precopulatory interaction with the female. Males rendered experimentally alkaloid-free fail to produce the pheromone and are les s successful in courtship. The male produces the pheromone in proporti on both to his alkaloid load and to the amount of alkaloid he transfer s to the female. The pheromone could thus serve as an indication of ma le ''worth'' and provide a basis for female choice. Utetheisa females are promiscuous and therefore are able to accrue multiple nuptial gift s (alkaloid and nutrient, both transmitted with the spermatophore). Th ey use sperm selectively, favoring those of larger males. Larger males in nature are also richer in alkaloid. Females therefore reinforce af ter copulation the choice mechanism they already exercise during court ship.