Comparative analysis of Anopheles gambiae larval and adult dorsal vessel structure and hemolymph circulation

To circulate hemolymph (blood) throughout their hemocoel (body cavity), insects employ an open circulatory system driven primarily by the contraction of a muscular dorsal vessel. In mosquito adults the dorsal vessel spans the length of the dorsal midline and contracts both toward the head (anterograde) and toward the posterior of the abdomen (retrograde). Because mosquitoes undergo drastic morphological changes during metamorphosis, hemolymph flow dynamics and dorsal vessel structure were investigated in larvae and compared to the adult stage. Intravital video recording of the larval dorsal abdomen showed that, unlike adults, the larval heart contracts solely in the anterograde direction, and does so at a rate of 1.7 Hz compared to 2.2 Hz in adults. Similarly, using particle tracking software, hemolymph was found to travel through the larval dorsal vessel at a significantly slower average velocity of 2 mm/s compared to 7 mm/s in adults. Fluorescence imaging of larval tissues associated with the dorsal cuticular epithelium following treatment with AlexaFluor-conjugated phalloidin (which binds F-actin) revealed a dorsal vessel that is comprised of an abdominal heart and a thoracic aorta. Ostia, or valve-like openings, are absent in the aorta, but an ostial pair is found at each of the abdomen’s seven intersegmental junctions, including one pair at the thoraco-abdominal junction. However, unlike adults, these ostia do not function as primary inlets of hemolymph flow into the lumen of the vessel, as hemolymph in larvae enters the dorsal vessel primarily through a distal incurrent opening located in the last abdominal segment.