An evolutionary treatment of the morphology and physiology of circulatory organs in insects.

An overview from an evolutionary perspective is presented on the research of the past 2 decades on insect circulatory organs. Based on various functional morphology it is clear that the flow mode of the dorsal vessel ('heart') has changed during the evolution of hexapods. In all apterygotes and mayflies the flow is bidirectional. In most pterygote insects, however, it is unidirectional. In some endopterygote insects, the direction of the flow alternates. This is achieved by heartbeat reversal, which may have various physiological functions and is a derived condition that probably occurred several times during the course of insect evolution. Special attention is given to the hemolymph flow in body appendages. In ancestral hexapods, they are supplied by arteries, whereas circulation in appendages of higher insects is accomplished by accessory pulsatile organs. These auxiliary hearts are autonomous pumps and exhibit a great diversity in their functional morphology. They represent evolutionary innovations which evolved by recruitment of building blocks from various organ systems and were assembled into new functional units. Almost all pulsatile circulatory organs in insects investigated exhibit a myogenic automatism with a superimposed neuronal control. The neuroanatomy of insect circulatory organs has been investigated only in a small number of species but in considerable detail. Numerous potential peptidergic and a few aminergic mediators could be demonstrated by immunocytochemical and biochemical methods. The cardiotropic effectiveness of these mediators may vary among species and it can be stated that there is no uniform picture of the control of the various circulatory organs in insects. A possible explanation for the differences may lie in the different evolutionary origins of the muscular components. Furthermore, insect circulatory organs may represent important neurohemal releasing sites.