Rhodnius prolixus is attracted to its host from short range mainly by the warm air diffusing from it, but partly by the smell of the skin. No evidence was obtained of either attraction or repulsion by moisture.
The olfactory and thermal senses reside chiefly in the antennae, which serve also to perceive air currents and contact. Under the conditions employed in our experiments, the location of the source of stimulus seems to be effected solely by the antennae, the eyes playing no part.
In the presence of two sources of stimulus, Rhodnius chooses one or the other and does not make for a point between the two. After removal of one antenna, it is still able to locate the source of stimulus in whatever direction this may lie. These observations are not compatible with orientation by the muscle tonus mechanism or tropotaxis.
The direction of the source of stimulus is usually inferred only after the antennae have been extended in every direction in turn, and it is suggested that orientation consists in a reflex pursuit of the antennae into the zone of optimal stimulation. This mechanism comes closest to the telotaxis of Kühn, or to the co-ordinated reflex mechanism of Mast. But it is pointed out that the factor of volition has not been definitely excluded.
After removal of both antennae, Rhodnius becomes inert and torpid, showing that these have a "kinetic" function. In the antenna-less insects, the feeding response can be readily elicited by moving objects, which are actively pursued. In antenna less insects with the eyes covered, the feeding response may be elicited by vibration. Vision and vibration may therefore be accessory factors in enabling the normal insect to find its host.
The structure of the antennae and their sensilla is described.