Gorging is stimulated in some insects by plasma components alone. In other insects engorgement is maximal only when blood cell components are present in addition to the plasma. Plasma components recognized by insects are NaCl, NaHCO3 (some anopheline mosquitoes) and albumin (Anopheles dirus and Pediculus humanus). Those requiring blood cells recognize this fraction by its adenine nucleotides which, for optimal engorgement, have to be offered in the presence of isotonic NaCl solution buffered by NaHCO3 (i.e. Rhodnius, culicine mosquitoes, Simulium, tsetse and Tabanus). The presence of albumin in the feeding medium may lower the level of nucleotides required to induce gorging (i.e. Aedes).
For most haematophagous insects requiring the cellular fraction, ATP is the most effective phagostimulant. It may however be replaced quite effectively by non-hydrolyzable analogues, indicating that ATP is not essential as a source of energy for the gustatory receptors involved in blood meal recognition.