Functional morphology of the dorsal vessel in the adult fly Protophormia terraenovae (Diptera, Calliphoridae)

The morphology and ultrastructure of the contractile tubular vessel acting as the cardiac pump in Protophormia terraenovae flies were analyzed by mean s of light microscopy, SEM and TEM. The results provide a novel anatomical picture of the two vessel portions, the abdominal heart and the aorta, and lay the foundations for an interpretation of the cardiocirculatory function in the fly. In the thorax, the thin and unchambered aorta is without apert ures, while the abdominal heart presents a very small caudal aperture and p airs of lateral ostia, one in each of the five chambers of which it is comp osed. The ostia of the four more distal chambers are of the incurrent type, which is to say that they act as valves ensuring that hemolymph flows only into the heart. Conversely, the ostia in the most proximal chamber allow h emolymph to flow both into and out of the heart. The entire vessel is compo sed of a single layer of myofibers that are oriented circularly around the lumen in the abdominal heart and longitudinally in the thoracic aorta. The abdominal heart has a thicker wall, a far more diffused and thick distribut ion of tracheoles, and a far greater number of mitochondria with respect to the aorta. This arrangement ensures a greater availability of oxygen and e nergy in the abdominal heart compared to the aorta and leads one to suppose that the high- and low-frequency contractions of the cardiac cycle (Thon, [1982] J. Insect Physiol. 28:411-416) can be attributed to the abdominal he art and the aorta, respectively. J. Morphol. 240:15-31, 1999. (C) 1999 Wile y-Liss, Inc.