Dorsal vessel structure and hemolymph circulation in Anopheles gambiae larvae

Insects transport nutrients, wastes, signaling molecules, and immune factors throughout their bodies using an open circulatory system driven primarily by a pumping organ called the dorsal vessel. To characterize the structure of the larval dorsal vessel in the malaria mosquito Anopheles gambiae, fluorescent images were acquired of the tissues associated with the dorsal cuticular epithelium following treatment with AlexaFluor-conjugated phalloidin, which binds F-actin. Fluorescence imaging revealed a muscular dorsal vessel that is comprised of an abdominal heart and a thoracic aorta. Ostia, or valve-like openings in the dorsal vessel, appear to be 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. Using particle tracking software, injected fluorescent latex microspheres were found to travel through the dorsal vessel at an average velocity of 2 mm s^-1, which is between one-third and one-fourth of that seen in adults. Moreover, intravital video recording of the dorsal abdomen showed that the resting larval heart contracts at a rate of 1.66 Hz, which is significantly lower than the adult heart rate. Finally, although the heart of adults contracts bidirectionally, the larval heart contracts exclusively in the anterograde direction, with the hemolymph entering the dorsal vessel primarily through an incurrent opening located at the posterior terminus of the heart and exiting via an excurrent opening at the anterior terminus of the aorta.