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

Abstract

To determine patterns of nymphal development, extensive collections were made throughout one complete year. Most nymphs had a life cycle of one year; the ultimate instar was reached in October and served as the overwintering stage. A small fraction of the population exhibited retarded growth and required two years to complete their development. Their first winter was spent in stages other than the final and their second winter in the final stage. Striking differences occurred in seasonal responses to photo-periods of 11 and 14 hours. For nymphs collected in Aug. and early Sept., final instar durations were much greater on the longer photo-period. In subsequent collections, however, the longer photoperiod induced more rapid development. This abrupt reversal in response coincided with the fall equinox. Differences in rate of response by nymphs maintained on the two photoperiods became progressively less as time of emergence approached. The threshold light intensity necessary to elicit a photoperiodic response was found to be extremely low, probably below 0.002 lux. More than 8, but less than 20, long-day cycles were necessary to induce irreversibly a response similar to that of the long-day controls.