This behavioral study was undertaken to understand and use M. sexta as a research animal, particularly in endocrinology. Time-lapse cinematography was used with laboratory-reared specimens to obtain information on and timing of behavioral activities reflecting physiological age. Only the most advanced insects were selected for study, thus avoiding timing irregularities involving poor vigor and the gating phenomenon. These advanced insects varied less than .+-. 0.9% SD from hatch through pupation. More than 20 events were timed and described, with emphasis on occurrences between the 5th ecdysis and pupation. At 25.degree. C, hatch occurred 4.0 days after oviposition (0 mean time = 0005 h), and ecdyses of the remaining 4 larval instars were at 6.4, 8.4, 10.6 and 13.7 days; pupation occurred at 22.8 days. The first 4 instars underwent a pre-ecdysis molt sleep (lasting 14-25 h, depending upon instar) in which they did not feed, defecate or move until ecdysis. Growth rate was highest for the 1st instar larvae and slowest for the 5th instar, with weight-doubling times of 11.3, 17.6, 11.4, 17.3 and 21.2 h for the 1st-5th instar feeding periods, respectively. Body wetting, found only in the last instar, was induced by the onset of the scotophase. Use of this event marker allowed the selection of a group of insects that were capable of pupating 5 days later within 1 h of each other.