Feeding responses of the horsefly, Tabanus nigrovittatus, to physical factors, ATP analogues and blood fractions

Wild-caught females of Tabanus nigrovittatus were presented with solutions of 7 analogues of ATP in 0.15 M NaCl, or various blood fractions, either as free liquids at 22 or 38°C or covered with a Parafilm M membrane at 38°C. Warming the diet, so that it could stimulate the insects’ heat receptors, or presenting it warmed and covered with a membrane, which the flies could pierce and thus deploy their mouthparts as they would when blood-feeding, enhanced the response to gorging stimulants. ADP (ED50 45 µM) was the most potent chemical phagostimulant. There were mo significant differences between the potencies of AMP, A(TETRA)P, AMP-PCP, AMP-PNP or AMP-S, which were 3.5-5 times less potent than ADP. Cyclic AMP had no phagostimulatory activity at concentrations of 400 or 1000 µM. The ED50 for washed red blood cells (RBC) in saline was 4.5% (one-tenth the concentration found in blood). RBC-free plasma caused only 10% gorging, but plasma with 0.5% RBC caused 61% gorging, indicating synergism between RBC and plasma.