Water Absorption by Eggs and Serosal Specialization as Clues to Evolutionary Trends in Heteroptera

Abstract

Water absorption by eggs, expressed by increase in fresh egg weight, was observed in Notonecta triguttata Say, Gerris lacustris latiabdominis Miyamoto, and G. gracilicornis gracilicornis Horvath. In these aquatic and semiaquatic bugs, whose eggs develop in water, serosal specialization at the posterior egg pole (columnar serosa) occurs. Close association was observed between onset of water absorption by the eggs and that of embryonic growth. Fresh weight of the developing eggs hardly changed in Hygia opaca Uhler, the eggs of which develop on damp ground, whereas it decreased steadily in Scotinophora lurida Burmeister and Coptosoma punctissimum Montandon, whose eggs develop on plant leaves. Serosal specialization at the posterior egg pole was never observed in these terrestrial bugs. Rearing of these heteropteran eggs under unusual cultural conditions and topical application of a microcauterizer at the columnar serosa in G. paludum insularis Motschulsky suggested that water absorption by the eggs was necessary to ensure enough oxygen to the embryos. Occurrence of serosal specialization and resulting water absorption by the eggs are considered from the viewpoint of heteropteran evolution.