Feeding behaviour of the horsefly Tabanus nigrovittatus (Diptera: Tabanidae): effects of dissolved solids on ingestion and destination of sucrose or ATP diets

Wild caught females of T. nigrovittatus were fed solutions containing either sucrose or ATP as a phagostimulant. The sucrose diets were presented as free liquids at room temperature. The ATP in the other diets was dissolved in 0.15 M NaCl, or in 0.3 M or 1 M cellobiose, and was presented covered with a Parafilm membrane at 38°C. The content of dissolved solids in some diets was adjusted with added cellobiose, which had no phagostimulatory effect. Diets of 1 M or 0.6 M sucrose were ingested in large amounts and were directed to the crop. Diets of 0.3 M or 0.1 M sucrose induced lower levels of ingestion, and the diet was directed to the midgut. The addition of cellobiose to give a 1 M total concentration of dissolved solids in these sucrose diets induced many insects responding to the lower concentrations of sucrose to direct the diet to the crop. Diets of ATP in saline or in 0.3 M cellobiose were directed to the midgut. ATP in 1 M cellobiose caused many of the insects to put the diet in both the crop and the midgut. Two behavioural models are presented that integrate interactions between the level of phagostimulation, the concentration of dissolved solids, and an endogenous feeding factor. The dissolved solids effect is most likely a response to osmotic pressure.