Blood as fuel: the metabolic cost of pedestrian locomotion in Rhodnius prolixus.

Active searching for vertebrate blood is a necessary activity for haematophagous insects, and it can be assumed that this search should also be costly in terms of energetic expenditure. Whether by swimming, walking, running or flying, active movement requires energy, increasing metabolic rate relative to resting situations. We analysed the respiratory pattern and energetic cost of pedestrian locomotion in the blood-sucking bug Rhodnius prolixus using flow-through respirometry, by measuring carbon dioxide emission and water loss before, during and after walking. We observed an increase in the metabolic rate during walking as compared with resting of up to 1.7-fold in male R. prolixus and 1.5-fold in females, as well as a change in their respiratory pattern, which switched from cyclic during rest to continuous when the insects started to walk, remaining in this condition during locomotion and for several minutes after stopping. Walking induced a significant loss of mass in both males and females. This can be explained by an increase in both metabolic rate and water loss during walking. These data constitute the first metabolic measures of active haematophagous insects and provide the first insights into the energetic expenditure associated with the active search for blood in this group.