Since the 1600s, when Malpighi first turned his microscope to the study of insect internal structures, insect biologists have found insects to contain a great diversity of unique structures. In the 16 and 1700s, many biologists searched in vain for insect arteries and veins analogous to the vertebrate circulatory system. The absence of blood vessels led many to conclude that insect blood did not circulate.
One of the first observations of insect blood circulation was from the microscopist Baker who described the use of a microscope to observe insect blood movement through wing veins. Baker’s work was expanded by Carl Gustav Carus, (The “Carus”) in the 1820s who not only observed blood flow, but reported its direction of movement. The idea of insect hemolymph circulation gained traction in the 1830s, but knowledge was slow to penetrate the entomological community. However, by 1847, the open circulatory system was well understood and a comprehensive account of the structure and function was published by the French scientist, M. V’erloren.
V’erloren, M. 1847. M’emoire sur la circulation dans les insecte:” Mem. Sav. e’tr. Acad. R. Belg. 79: 7-93.