Neuroendocrine regulation of migration and reproduction in the grasshopper Melanoplus sanguinipes fabricius
This study examines the endocrine regulation of migration and reproduction in the migratory grasshopper, Melanoplus sanguinipes. Even though, in many insects, migration is considered as energetically costly and seems to extract a cost in terms of reproduction, this species has shown to exhibit enhanced reproduction after performance of even one long, tethered flight to exhaustion. This study sought to understand the physiological mechanisms underlying this enhanced reproduction after long flight. The lipid content of eggs of long fliers flown to exhaustion was not different from that of non fliers. In contrast, protein content of eggs was significantly lower in long fliers flown to exhaustion compared to non fliers and 1 hr fliers. Titer determinations of hemolymph AKH (Adipokinetic Hormone) were done at rest and after various periods of flight. No significant differences in AKH levels in CC (Corpora Cardiaca) were detected between non-migrants, animals that had flown for one hour to identify them as migrants, and animals that had flown to exhaustion. Adipokinetic response to AKH I was greater in migrants than in non-migrants. JH (Juvenile Hormone) levels increased on days 5 and 8 in animals flown to exhaustion on day 4 but not in one-hour or non-flier controls. Treatment of grasshoppers with JH III induced early reproduction as did performance of long duration flight. This suggests that increased JH titer after performance of a long-duration flight is at least one component of flight-enhanced reproduction. AKH treatment had no effect on JH titers. There was no difference in the amount of peptide in the CC after long duration flight. However, there were differences in the amount of peptides in hemolymph, and partial sequence information of the peptides revealed that it is identical to that of serine protease inhibitor in locusts. No difference in mating frequency or duration after long flight was observed between non fliers and long fliers. However, females after long flight consumed significantly greater quantities of food and displayed increased digestive capacity for lipid. These results suggest that nutritional compensation by changes in feeding behavior and food utilization could partially account for increased reproduction after long flight in M. sanguinipes.