Effects of intraspecific competition on the life cycle of the stonefly, Nemurella pictetii (Plecoptera: Nemouridae)

Intraspecific competition had important influence on growth and development of the larvae. High larval densities led to reduced growth and retarded development through interference, not through exploitative competition. All specimens were negatively affected by frequent encounters and the resulting disturbance. There were no dominant individuals able to grow and develop faster than the rest, at the expense of the others.Differences in life cycle length of Nemurella pictetii may result from different larval densities in different microhabitats and resultant different degrees of interference competition. Although competition alone probably does not cause splitting of populations into cohorts with different life cycle duration differences in size and development caused by other factors are certainly enhanced by intraspecific competition.Flexible life cycle duration is widespread in insects [1,2], also among aquatic species [3]. Variation may concern an important portion of normal life cycle length. It arises mainly from differences in the duration of the larval stage in which most of the life cycle is spent [4]. The main causes for such differences are different temperatures, day length or food in the various habitats of the population (see, for example, the review by [5]).Differences in life cycle length between individuals of a population at the same site have also repeatedly been reported for aquatic insects [6-10], but the causes are less easily explained. In the case of extended hatching from eggs, early hatching individuals are often assumed to have an advantage in development resulting from the combined effects of temperature and longer growth period [11-13]. Such explanations focus mainly on time as factor. However, what causes a population to split into several cohorts growing and developing at different speed? There are no satisfactory explanations so far. Often, cohort splitting occurs when larval density at the respective site is high (see below). Intraspec