TEMPERATURE-DEPENDENT GROWTH AND LIFE-CYCLE OF DELEATIDIUM (EPHEMEROPTERA, LEPTOPHLEBIIDAE) IN 2 HIGH-COUNTRY STREAMS IN NEW-ZEALAND

1. Measurements of larval growth rates were used to produce an empiric al model for examining factors influencing patterns of size structure and emergence period for populations of the leptophlebiid mayfly Delea tidium (lillii group) in two high-country streams (South Island, New Z ealand). Growth rates were measured in situ by enclosing groups of lar vae and natural stream substrata in growth chambers. 2. Multiple regre ssion analysis showed that temperature explained most variation among growth rates in both streams. Growth rates were also significantly rel ated to larval size, although the effect was minor compared with tempe rature. A significant relationship between larval biomass per chamber and growth rate was shown at only one site. 3. Growth models based on multiple regression equations (R(2) = 0.70-0.84) simulated the extende d emergence period and complex size structure observed for populations of Deleatidium in the field. Larvae hatching from eggs deposited befo re mid-February (austral summer) probably emerge as adults before May (minimum cohort duration approximate to 3 months). Larvae hatching aft er mid-February probably do not emerge as adults until the following s ummer because of low growth rates during winter (maximum cohort durati on approximate to 11 months). 4. On average, there are probably two ge nerations of Deleatidium per year (bivoltine) at the study sites. The presence of numerous overlapping cohorts throughout summer, however, r esults in an extended emergence period and complex size structure.