A family of myotropic neuropeptides sharing the common C-terminal pentapeptide Phe-Xxx-Pro-Arg-Leu-NH2 (Xxx = Ser, Thr, Val), known as the pyrokinins, has been isolated from the cockroach Leucophaea maderae and locust Locusta migratoria of the order Orthoptera. A hormone (Bom-DH) that elicits diapause induction in the silkworm Bombyx mori (order Lepidoptera) also contains this C-terminal pentapeptide (Xxx = Gly). The orthopteran pyrokinin neuropeptides elicit significant diapause-inducing activity in the lepidopteran silkworm. Despite containing the sterically bulky, inflexible Val residue in the variable Xxx position, the locust pyrokinin Lom-PK is threefold more active than native Bom-DH as a diapause induction agent. The C-terminally truncated cockroach leucopyrokinin (LPK) fragment, Thr-Ser-Phe-Thr-Pro-Arg-NH2 [LPK(2-7)], proved virtually inactive in the silkworm assay, demonstrating the importance of an intact C-terminal pentapeptide sequence to diapause induction activity. Bom-DH also elicits significant myostimulatory activity in a cockroach hindgut assay, although at a level several orders of magnitude less than the native myotropic peptide LPK. However, the C-terminal pentapeptide of Bom-DH (Xxx = Gly) is equipotent with the LPK C-terminal pentapeptide (Xxx = Thr) as a myostimulatory agent. The cross-activity observed for the various pyrokinins suggests that the receptors that mediate the disparate physiological processes of diapause in the silkworm and hindgut contraction in the cockroach share homologous features. Previous data suggest that pyrokinins demonstrate cross-activity in silkworm pheromone production, locust/cockroach oviduct myotropic assays, and in armyworm cuticular melanization. These results, combined with the present work, increase the known spectrum of cross-activity for this insect neuropeptide family to include five different physiological processes.