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 cutic ular lipids to insect water balance mere investigated in natural population s of the grasshopper Melanoplus sanguinipes. I specifically test the hypoth eses that patterns of discontinuous ventilation affect water loss and that increased amounts and melting points of cuticular lipid reduce water loss. Using how-through respirometry, rates of water loss and carbon dioxide rele ase from grasshoppers mere quantified at 25, 35 and 42 degrees C. Populatio ns displayed substantial variation, with high-elevation populations exhibit ing the greatest mater loss and metabolic rates. Behavior leading to discon tinuous gas exchange was observed in several populations, but its occurrenc e decreased dramatically at high temperatures and was not correlated with a reduction in the rate of water loss. The amount and melting point of cutic ular lipids were determined for each individual using gas chromatography an d Fourier-transform infrared spectroscopy. Increased amounts and higher mel ting points of cuticular lipids were strongly correlated with lower rates o f water loss in populations. I show that discontinuous gas exchange is unli kely to be a mechanism for reducing water loss in these insects and that th e lipid properties are primarily responsible for variation in overall water loss rates.