Abstract Triatomines (Heteroptera, Reduviidae) are vectors of Trypanosoma cruzi , the etiological agent of Chagas disease in America. These true bugs have traditionally been considered to be blood suckers, although some species have been catalogued as being entomophagous. By using their highly specialized mouthparts, these insects have evolved a stereotyped habit which includes lifting up the proboscis, piercing and sucking, when the occasion arises. Most triatomines bite their sleeping and unaware vertebrate or invertebrate hosts, but they can also search for other targets, guided, in part, by visual and chemical stimuli. In this study, we observed that triatomines apparently visually identify a drop of water in the distance, then taste it with their legs, upon which proboscis extension and sucking ensues. This invariant behavior or fixed action pattern, observed in several triatomine species ( Rhodnius prolixus , Triatoma infestans and Panstrongylus geniculatus ), was also elicited by a dummy drop of water and guava fruit. We discuss evolutionary and ethological aspects of this innate behavior. Digital video images related to this article are available at http://www.momo-p.com/showdetail-e.php?movieid=momo180314rp01a and http://www.momo-p.com/showdetail-e.php?movieid=momo180314rp03a .