Blood is a remarkable material: it must remain fluid inside blood vessels, yet clot as quickly as possible outside them, to stop bleeding. The chemical cascade that makes this possible is well understood for vertebrate blood. But hemolymph, the equivalent of blood in insects, has a very different composition, being notably lacking in red blood cells, hemoglobin, and platelets, and having amoeba-like cells called hemocytes instead of white blood cells for immune defense.