Haemolymph sugars and the control of the proventriculus in the honey bee Apis mellifera

Crop emptying and rectal filling rates were investigated in bees trained to collect defined amounts of sucrose solution. Crop emptying rates strongly depended on the sucrose concentration of the collected solution. There was a close match between the energy expenditure of the bees and the amount of sucrose transported through the proventriculus, irrespective of the fluid dilution. Results indicated that the controlling variable was the amount of sucrose flowing through the proventriculus rather than the volume flow. In order to distinguish between haemolymph osmolality and haemolymph carbohydrate levels as factors controlling the activity of the proventriculus, bees were injected with either metabolizable or non-metabolizable carbohydrates. Only the injection of metabolizable carbohydrates modulated the activity of the proventriculus, indicating that the titers of metabolizable carbohydrates are involved in the feedback loop controlling crop emptying, and that haemolymph osmolality alone does not influence the activity of the proventriculus.