Geographic and Altitudinal Variation in Water Balance And Metabolic Rate in a California Grasshopper, Melanoplus Sanguinipes

The importance of respiratory patterns and the physical properties of cuticular lipids to insect water balance were investigated in natural populations of the grasshopper Melanoplus sanguinipes. I specifically test the hypotheses that patterns of discontinuous ventilation affect water loss and that increased amounts and melting points of cuticular lipid reduce water loss. Using flow-through respirometry, rates of water loss and carbon dioxide release from grasshoppers were quantified at 25, 35 and 42 degrees C. Populations displayed substantial variation, with high-elevation populations exhibiting the greatest water loss and metabolic rates. Behavior leading to discontinuous gas exchange was observed in several populations, but its occurrence decreased dramatically at high temperatures and was not correlated with a reduction in the rate of water loss. The amount and melting point of cuticular lipids were determined for each individual using gas chromatography and Fourier-transform infrared spectroscopy. Increased amounts and higher melting points of cuticular lipids were strongly correlated with lower rates of water loss in populations. I show that discontinuous gas exchange is unlikely to be a mechanism for reducing water loss in these insects and that the lipid properties are primarily responsible for variation in overall water loss rates.