Rhodnius has a sucking tube that it can use to sip saliva from the skin surface or stick into the skin to drink blood. It will also feed on artificial solutions in the laboratory. Liquids are pumped by the pharynx into the insect. As the liquid passes the pharynx, it passes taste receptors responsive to nutrients in the liquid. The energy molecule, ATP, is detected and will act as a feeding stimulant. Receptors measure the concentration and ratios of sodium and potassium salts in the liquid. Only salt concentrations between 0.1 and 0.15 M trigger feeding. This is the concentration range of these salts in blood. Liquids with high salt concentration (> 3M) are rejected as are liquids that lack salts. Salts are monitored continuously during feeding. A sudden change in salt concentration outside the acceptable range will terminate feeding.
Rhodnius depends on blood for survival. It has evolved receptors and sensory feedback that trigger feeding on liquids that mimic blood and reject liquids that are not similar enough to blood. Rhodnius is adapted to engorge on blood but drink little of other liquids.
Gina Pontesa, Marcos H. Pereirab & Romina B. Barrozoa. Salt controls feeding decisions in a blood-sucking insect. Journal of Insect Physiology Volume 98, April 2017, Pages 93–100.