LIFE-HISTORY AND PHOTOPERIODIC RESPONSES OF NYMPHS OF TETRAGONEURIA CYNOSURA (SAY)

Abstract

1. Most nymphs of Tetragoneuria cynosura required one year for the completion of nymphal development in nature. Rapid growth occurred in the summer months and by the end of October, the final instar was attained which served as an overwintering diapause stage; emergence occurred in early April and May. A small percentage of the population (5-10%) exhibited growth retardation and had a lifecyle of two years. These individuals spent the first winter in stages short of the last, the summer in the penultimate stage, and the second winter in the last stage.2. Striking differences occurred in seasonal response to photoperiods of 11 and 14 hours. Durations of the final instar for nymphs collected in August and September were much greater on the longer day-length. Following the fall equinox period, a reversal in response occurred, with the longer photoperiod inducing more rapid development in fall and winter collections. Differences in rates of response became progressively less as time of emergence approached. These results were utilized in an attempt to explain the role of photoperiod in controlling seasonal nymphal development in this species.3. The threshold light intensity necessary to elicit a photoperiodic response was found to be extremely low, probably below 0.002 lux. More than eight, but less than twenty, long-day cycles were necessary to induce irreversibly a response similar to that of the long-day controls.