MILLER, WA; HATLE, JD; BORST, DW: The second half of the molt cycle in lubber grasshoppers is developmentally inflexible

To deal with unpredictable environments many animals remain developmentally flexible, becoming unresponsive to environmental change (canalized) after commitment to a specific developmental pathway. We tested the hypothesis that Romalea microptera exhibit a canalized phase at the end of the molt cycle, during which the process of molting does not respond to food availability. Fifth instar males were tested on one of five food quantity treatments: 1) high-fed throughout the instar (H); 2) high switched to low at 13 d of the instar (HL13); 3) high switched to low at 8 d (HL8); 4) high switched to low at 3 d (HL3); 5) low-fed throughout (L). We collected hemolymph samples daily starting at 5 d and measured ecdysteroids (arthropod molting hormones) by RIA. Diet affected age at adult eclosion (H=15.4�0.5 d; HL13=16.4�0.5 d; HL8=15.8�0.7 d; HL3=26.8�3.5 d; L=26.5�1.5 d). Likewise, diet affected age at the maximum level of ecdysteroids (Emax), and age at maximum mass such that H = HL13 = HL8 < HL3 = L. In contrast, diet had no effect on the maximum level of ecdysteroids, the time from Emax to eclosion (H=4.7�0.3 d; HL13=4.7�0.3 d; HL8=4.5�0.3 d; HL3=5.4�0.3 d; L=4.5�0.3 d) and a relatively small effect on time from maximum mass to eclosion (H=5.1�0.4 d; HL13=6.0�0.4 d; HL8=7.1�0.5 d; HL3=3.7�0.5 d; L=3.7�0.5 d). Our data demonstrate that Emax occurs during the canalized phase. Furthermore, a limiting diet starting at 8 d did not affect the age at eclosion (which is about 16 d), so the canalized phase must begin during the first half of the molt cycle in well-fed animals. We hypothesis that lubbers pass an as yet unknown threshold between 3 d and 8 d, and this causes ecdysteroid titers to increase and stimulate molting. (Supported by NSF grant DBA-9978810 to DWB).