Many organisms exhibit developmental plasticity only in sensitive phases and cannot respond to environmental perturbations at other times. However, we know little about the physiological events that define plastic and canalized phases. During egg production in insects, vitellogenin (Vg) accumulates first in the hemolymph and then in the eggs. In addition, storage proteins may be important resources for egg production. Therefore, we tested hypotheses on the relationships of Vg and TP (total hemolymph protein minus Vg) titers to the transition from flexible to inflexible development during egg production. In lubber grasshoppers, approximately 70% of TP is contained in three proteins that range from 68 to 83 kDa. We maintained females on food treatments that produced defined plastic and canalized periods, collected hemolymph every approximately 4 d, and determined the ages at which oviposition and the maximum Vg and TP titers occurred. Both Vg(max) titer and especially TP(max) titer were predictors of the number of eggs produced. The time from eclosion to Vg(max) was significantly affected by diet, but the time from Vg(max) to oviposition was not. Similarly, the time from eclosion to TP(max) was significantly affected by diet, while the time from TP(max) to oviposition was not. Hence, Vg(max) and TP(max) are physiological landmarks that occur during the canalized phase of egg production.