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. Either if it is by swimming, walking, running or flying, active movement requires energy, increasing metabolic rates relative to resting situations. We analysed the respiratory pattern and the 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 of up to 1.7-fold in the metabolic rate during walking as compared to resting in male R. prolixus and 1.5-fold in females, as well as a change in their respiratory pattern. The last switched from cyclic during resting to continuous, when the insects started to walk, remaining this condition unchanged during locomotion and for several minutes after stopping. Walking induced a significant loss of weight in both, males and females. This can be explained by the increase in both, the metabolic rate and the water loss during walking. These data constitute the first metabolic measures of active hematophagous insects and provide the first insights on the energetic expenditure associated to the active search for blood in this group.