Ventilatory mechanism and control in grasshoppers

Grasshoppers exhibit a diversity of ventilatory patterns depending on activity status. For each pattern, the mechanism and control of gas exchange is analyzed in terms of a two-step model, consisting of tracheolar and trans-spiracular steps in series. During the intermittent gas exchange that characterizes the most quiescent grasshoppers, spiracles open and close in response to changing carbon dioxide, and trans-spiracular resistance controls gas exchange. In resting but alert grasshoppers, abdominal pumping occurs, and gas exchange is controlled equally by tracheolar and trans-spiracular resistances; trachea! oxygen and carbon dioxide are regulated by variation in abdominal pumping and spiracular opening. During hopping, abdominal pumping does not occur, and bulk gas flow is driven by cuticular deformations associated with locomotion. Increased cellular oxygen consumption depends on use of internal oxygen stores and increased partial pressure gradients. After hopping ceases, abdominal pumping increases dramatically and restores trachéal gas composition; however, the rise in abdominal pumping after hopping is not affected by trachéal gas levels. During flight, bulk flow to the flight muscles is driven by tidal thoracic auto-ventilation, while the remainder of the body is ventilated by abdominal pumping. During both hopping and flight, the greatest resistances to gas transport exist in the tracheolar rather than the trans-spiracular step.