Heat is the principal host-associated cue for the blood-sucking bug Rhodnius prolixus. It is both necessary and sufficient to trigger the ''proboscis extension response'' (PER), an essential element of the feeding behaviour of this insect. The aim of this study was to determine whether the temperature of an object itself or the thermal contrast between the object and the environmental background is responsible for triggering the PER. Thermal stimuli at 25-50 C were presented in thermal environments of 20-40 C. The results showed that stimuli at 30 and 35 C trigger the highest rates of response, provided that they were presented in thermal backgrounds at temperatures of below 35 C. Thus, bugs display a preference for objects at temperatures corresponding to those at the surface of their vertebrate hosts (birds and mammals). However, this preference disappears if no heat exchange between the bug and its potential host is possible (i.e. if they are at the same temperature) and may even become negative if the insect and the surrounding environment are at a temperature above 35 C). In these situations, and when the object was too warm to be a potential host, PER rates were much lower. These findings have potential implications for the feeding strategies adopted by triatomine bugs in the natural tropical areas they inhabit.