Sex pheromones have been chemically identified for scale insects (Margarodidae and Diaspididae), mealybugs (Pseudococcidae), and aphids (Apididae) among Homoptera and for plant bugs (Miridae), assassin bugs (Reduviidae), stink bugs (Pentatomidae), and shield bugs (Scutelleridae) in Heteroptera. The pheromone chemistry of Hemiptera is diverse, reflecting the extent of radiation of the group, and most hemipteran sex pheromones have been exploited by parasitoids as host-finding kairomones. Pheromones are produced by males in taxa characterized by large individuals, and by females in species having relatively small individuals. The known examples of acoustical communication in Hemiptera corroborate the pattern of large body size favoring males as the attractive sex. It is suggested that risk of discovery by natural enemies provided the selective driving force for evolution of this pattern.