The evolutionary role and fate of the primary ovipositor in insects

The development of a piercing-sawing ovipositor for introducing eggs into living plant tissues has made its owners independent of the soil characteristics and increased egg protection. This was the most important prerequisite for the appearance of wings and flight which provided the winged insects with tremendous opportunities for finding new niches and led to unparalleled adaptive radiation. The ovipositor has passed several stages of improvement and differentiation. The four principal types of the primary ovipositor are considered: those of Odonata, Diaphanopteroidea (only extinct forms), Cicadina (including Paraneoptera and Hymenoptera), and Orthoptera. A new hypothesis of the gonangulum homologies is put forward, interpreting it as half of sternite IX lateral of the midline plus the paratergite of the same segment. The constructions of the valvae and different homologies of the third valvae in Polyneoptera and Eumetabola are discussed. The primary ovipositor has been repeatedly (i.e., in many lineages) reduced in the evolution of Pterygota. The main circumstances of these reductions are: (1) subterranean (fossorial) life in narrow cavities; (2) aquatic life of the larvae, mostly linked with submerged oviposition; (3) development and perfection of flight to which the heavy and protruding ovipositor was a hindrance. All the holometabolous insects except Hymenoptera lack the primary ovipositor.