Accessory pulsatile organs: evolutionary innovations in insects

In addition to the dorsal vessel ("heart"), insects have accessory pulsatile organs ("auxiliary hearts") that supply body appendages with hemolymph. They are indispensable in the open circulatory system for hemolymph exchange in antennae, long mouthparts, legs, wings, and abdominal appendages. This review deals with the great diversity in the functional morphology and the evolution of these accessory pulsatile organs. In primitive insects, hemolymph is supplied to antennae and cerci by arteries connected to the dorsal vessel. In higher insects, however, these arteries were decoupled and associated with autonomous pumps that entered their body plan as evolutionary innovations. To ensure hemolymph supply to legs, wings, and some other appendages, completely new accessory pulsatile organs evolved. The muscular components of these pulsatile organs and their elastic antagonists were recruited from various organ systems and assembled to new functional units. In general, it seems that the evolution of accessory pulsatile organs has been determined by developmental and spatial constraints imposed by other organ systems rather than by changes in circulatory demands.