Extracardiac versus cardiac haemocoelic pulsations in pupae of the mealworm (Tenebrio molitor L.).
Pulsations in mechanical pressure of the pupal haemocoele were investigated by means of simultaneous recording from multiple sensors. It has been determined that cardiac and extracardiac haemocoelic pulsations are each regulated by substantially different and quite independent physiological mechanisms. At the beginning and in the middle of the pupal interecdysial period the anterograde heartbeat and extracardiac pulsations occur in similar, but not identical periods. During the advanced pharate adult stage, there appear almost uninterrupted pulsations from different sources: cardiac, extracardiac, intestinal, and the ventral diaphragm.Extracardiac pulsations are associated with pressure peaks of 200-500 Pa, occurring at frequencies of 0.3-0.5 Hz. The effect of heartbeat on haemocoelic pressure is very small, 100- to 500-fold smaller, comprising only some 1 or 2 Pa during the vigorous anterograde systolic contractions. Accordingly, extracardiac pulsations are associated with relatively large abdominal movements from 30-90 µm whereas heartbeat produces movements of only 100-500 nm. This shows that extracardiac pulsations can be easily confused with the anterograde heartbeat. It does not seem realistic to assume that the relatively weak insect heart, and not the 100- to 500-fold more powerful extracardiac system of abdominal pump, could be at all responsible for selective accumulation of haemolymph in anterior parts of the body, for inflation of wings or enhancement of tracheal ventilation.It has been established that thermography from the pericardial region is not specific for the heartbeat. It records subepidermal movement of haemolymph resulting from the actions of both dorsal vessel and extracardiac pressure pulses as well. Shortly before adult eclosion the cardiac and extracardiac pulsations occasionally strike in concert, which profoundly increases the flow of haemolymph through pericardial and perineural sinuses. The relatively strong extracardiac pulsations cause passive movements of various visceral organs, tissue membranes, or tissue folds, giving thus a false impression of an authentic pulsation of tissues. In addition, extracardiac pulsations cause rhythmical movements of haemolymph between various organs, thus preventing haemolymph occlusion at the sites where the heart does not reach. It has been emphasized, finally, that the function of the autonomic nervous system (coelopulse), which integrates extracardiac pulsations, depends on homeostatic moderation of excessive or deficient conditions in insect respiration and haemolymph circulation.
Sláma
K
Institute of Entomology, Czech Academy of Sciences, Drnovská 507, 16100, Praha, Czech Republic