Figure 7.

Co-adaptation of the insect circulatory and immune systems, as exemplified in mosquitoes. Hemocytes exist in two states: in circulation and in sessile form. In naïve mosquitoes (top), some of the sessile hemocytes are aggregated around the ostia (valves) of the heart, and are called periostial hemocytes. Hemolymph flow is swift in the regions surrounding the ostia (the periostial regions), and the majority of hemolymph enters the heart through the ostia located in abdominal segments 4, 5, and 6. Upon infection (bottom), circulating hemocytes undergo mitosis and increase in number, and many hemocytes migrate to, and aggregate in, the periostial regions of the heart where they phagocytose pathogens. The periostial regions are the only location of the body where sessile hemocytes increase in number in response to infection. Infection does not significantly alter the proportion of hemolymph that flows through each periostial region, and periostial hemocytes – including their immune activity – preferentially aggregate around the ostia that experience the most hemolymph flow. Figure from Sigle and Hillyer (Sigle and Hillyer, 2016); reproduced with permission.