Fig. 4. Correlation between natural ecdysteroid titres and peptide production in Inka cells. (A) Total pre-ecdysis-triggering hormone (PETH)-immunoreactivity and ecdysis-triggering hormone (ETH)-immunoreactivity (IR) were low following ecdysis into the fifth instar, but increased considerably after the two ecdysteroid peaks appeared in the haemolymph and reached their highest levels on day 9. Total ETH-IR represents the levels of active ETH plus all precursor forms containing this peptide. The ratio of ecdysone to 20-hydroxyecdysone (20E) in the second (prepupal) peak was 1:20. (B,C) Elevation of ecdysteroid levels in the haemolymph was associated with a marked increase of the levels of isolated active hormones (PETH and ETH), and their precursor forms (PE, EA, PEA) in Inka cells. (B) Levels of isolated PETH were higher than those of ETH because some of the ETH contained the precursor EA. Levels of the two precursors (PE and PEA) decreased before pupal ecdysis on days 8–9, indicating peptide processing (C). Each ecdysteroid and peptide determination represents the mean ± S.D. of 4–11 samples, but the standard deviations were too small to be included in the figure. Note the difference in scale in C.