Fig. 7. The appearance of central nervous system (CNS) sensitivity to ecdysis-triggering hormone (ETH) associated with natural ecdysteroid pulses before larval and pupal ecdysis. Pharate fifth-instar larvae and pharate pupae become sensitive to ETH injection approximately 30 and 48 h before natural ecdysis, respectively (–30 and –48 h, arrowheads) when ecdysteroids reach peak levels. Grey areas depict the periods in pharate larvae (–28 to –20 h) and pharate pupae (–46 to –20 h) with increased ecdysteroid levels during which ETH injection induces the ecdysis behavioural sequence. Since the old cuticle is not sufficiently digested at this time, these animals fail to ecdyse. However, their ecdysteroid levels are sufficient to recover CNS sensitivity to ETH. After steroids levels decline 1 day later, the natural release of Inka cell peptides activates the entire behavioural sequence, and most animals ecdyse normally at the expected time (short arrow). Black areas depict the periods (pharate larvae at –10 to –6 h and pharate pupae at –12 to –4 h) during which ETH triggers strong and long-lasting ecdysis contractions, but these animals fail to ecdyse. Since, at these stages, ecdysteroid levels are too low to induce CNS sensitivity to Inka cell peptides, these animals never resume pre-ecdysis behaviour and remain trapped in the old cuticle. See text and Table 2 for details. Relative ecdysteroid levels are from itan et al. (itan et al., 1999) and the present study.