Temporal Patterns of Emergence in Aquatic Insects
In aquatic insects, emergence (ecdysis to the adult or subimaginal stage) varies widely in temporal pattern. The comparative study of this pattern is feasible and informative in orders such as Plecoptera, Ephemeroptera, Odonata, Diptera and Trichoptera in which all members of a population pass through the water-surface when emerging. Methods by which emergence rate can be measured are discussed. Four basic temporal patterns of emergence exist. Emergence may be (1) continuous with irregular fluctuations in rate; (2) rhythmic, with a lunar period; (3) sporadic, occurring at irregular intervals of a few days; or (4) seasonal. Examples of each of these patterns are given, and reference is made to the proximate and ultimate environmental factors which may be maintaining the patterns observed. Diurnal rhythms of emergence are excluded from consideration. When emergence is restricted seasonally in temperate latitudes, the degree of its synchronization within the emergence period varies widely but is usually constant and typical for a given species. This has provided the basis for an ecological classification of British Odonata, the validity of which is examined in the light of recent research.