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Canada L9G 3N6), andAbstract. The dorsal vessel of the blood feeding insect, Rhodnius prolixus, was found to increase or decrease its rate of contraction in response to a number of different stimuli. Handling increased contraction rates whereas tactile stimulation of the ventral abdominal cuticle inhibited contraction. Injection of very low concentrations of serotonin or of high concentrations of octopamine enhanced the inhibitory effect, apparently by acting via the nervous system. Higher concentrations of serotonin increased heart rate by acting directly on the myocardium. The inhibitory response is suggested to be one facet of a generalised thigmotactic response.Abstract. The gracilis muscle was excised from cold-acclimated rats, placed in vitro, and simultaneously perfused via its artery by high pO 2 medium and superfused by low pO 2 medium. With a doubling of the perfusion rate (from 50 to 100 ~tl/min) phosphocreatine and ATP increased by 39 % and 44 %, respectively.

The dorsal vessel (DV) in the abdomen of the blood-feeding insect Rhodnius prolixus was divided functionally into two regions, the heart, into which haemolymph entered the DV through four pairs of ostia located in abdominal segment VII, and the aorta, along which the haemolymph was propelled from abdominal segment VI to the thorax. Osmium-fixed whole mounts revealed the DV to consist of spirally arranged striated muscle fibers and to possess two rows of ventrally attached longitudinal fibers extending the length of the abdomen. Seven pairs of alary muscles were found attached to the DV in the posterior abdominal segments. Contractions of the alary muscles attached to the ventral surface of abdominal segments VII and VIII served to expand the heart. Electron microscopy revealed the DV to consist of a thin layer of contractile elements surrounded by an inner (intima) and outer (adventitia) connective tissue layer. Embedded in the intima along each lateral side of the DV were two large groups of endocardial cells extending the length of the DV. A small group of pericardial cells was embedded in the adventitia along the mid-ventral side of the DV, and clusters of pericardial cells were found attached to the alary muscles. Nerve terminals were found only on the heart: they contained agranular synaptic vesicles approximately 30 nm in diameter and densely stained granules approximately 100-120 nm in diameter. These structural components are discussed in relation to the role of the DV in circulation.

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