Photoperiodic response during induction of larval hibernal diapause of Chymomyza costata was characterized and the course of diapause development was analyzed in the laboratory. C. costata becomes sensitive to photoperiodic stimuli during an unspecified stage of its early development (embryo, 1st larval instar); the sensitivity gradually increases during the 2nd and early 3rd larval instars and reaches its maximum just before the moment when it abruptly ceases at the age of 15-19 days after oviposition. Diapause intensifies during a period of 2-3 weeks after induction and, later, is maintained without apparent development until death (between 150 and 250 days) under 18 degrees C and a short-day photoperiod (L10:D14, SD). Diapause may be terminated in a horotelic process by exposure to a low temperature (2 degrees C) during which larvae subsequently (1) synchronize their post-diapause development (requires up to 14 days of chilling), (2) lose photoperiodic sensitivity (2 months), and finally (3) terminate diapause (5 months). Alternatively, diapause may be terminated in a tachytelic process by exposure to a high temperature (18 degrees C) and long-day photoperiod (L16:D8, LD) during which no synchronization occurs and pupariation takes place after a mean of 25.2 days (with a broad range from 8 to more than 50 days). Larvae that are transferred from LD to SD during their sensitive period switch their developmental programming from pupariation to diapause. Proliferation of adult primordial structures (imaginal discs, neuroblasts) slows down within 1 day after transfer. In contrast, whole body growth continues for at least 3 days before its rate slows down and matches the rate characteristic for SD conditions.