10.2. The evolution of aquatic lifestyles
Hypotheses concerning the origin of wings in insects (section 8.4) have different implications regarding the evolution of aquatic lifestyles. The paranotal theory suggests that the “wings” originated in adults of a terrestrial insect for which immature stages may have been aquatic or terrestrial. Some proponents of the preferred exite—endite theory speculate that the progenitor of the pterygotes had aquatic immature stages. Support for the latter hypothesis appears to come from the fact that the two extant basal groups of Pterygota (mayflies and odonates) are aquatic, in contrast to the terrestrial apterygotes; but the aquatic habits of Ephemeroptera and Odonata cannot have been primary, as the tracheal system indicates a preceding terrestrial stage (section 8.3).
Whatever the origins of the aquatic mode of life, all proposed phylogenies of the insects demonstrate that it must have been adopted, adopted and lost, and readopted in several lineages, through geological time. The multiple independent adoptions of aquatic life-styles are particularly evident in the Coleoptera and Diptera, with aquatic taxa distributed amongst many families across each of these orders. In contrast, all species of Ephemeroptera and Plecoptera are aquatic, and in the Odonata, the only exceptions to an almost universal aquatic lifestyle are the terrestrial nymphs of a few species.
Movement from land to water causes physiological problems, the most important of which is the requirement for oxygen. The following section considers the physical properties of oxygen in air and water, and the mechanisms by which aquatic insects obtain an adequate supply.