Nectar feeding by the hovering hawk moth Macroglossum stellatarum: intake rate as a function of viscosity and concentration of sucrose solutions

Although nectar feeding in insects has long been studied. the knowledge of the effect of nectar energy content on the ingestion dynamics separately fr om the viscosity of the fluid is very limited. To determine the effects of both factors on the feeding behavior Of the hovering hawk moth Macroglossum stellatarum, we developed a method to independently manipulate sucrose con centrations and viscosity. The intake rate was analyzed as a function of su crose concentration, the concentration at constant viscosity (kept constant by adding tylose, an inert polysaccharide), and of the different viscositi es of a 30% weight/weight (w/w) sucrose solution (by adding different amoun ts of tylose). By increasing the concentration. and thus its viscosity, the solution intake rate (in mul s(-1)) decreased beyond a 20% w/w sucrose sol ution. For a 30% sucrose solution, the intake rate decreased with increasin g viscosity. At constant viscosity, the solution intake rate decreased beyo nd a 30% w/w sucrose solution. However. if we considered the quantity of su crose ingested per unit time (sucrose intake rate), the same fitted maximum was attained for both series in which the sucrose concentration changed (3 3.6% w/w). Results suggest that the gustatory input affects the dynamics of fluid ingestion separately from the viscosity.