the "blood" of the insect, but it is a colorless or slightly yellow-tinted lymph. It is kept in motion, however, by a pulsating vessel, or heart, lying
Fro. 69. Diagram of the typical structure of an insect's heart and supporting dia- phragm, with the course of the circulating blood marked by ,?o, aorta, or anterior tubular part of the heart without lateral openings; Dpk, mem- branous diaphragm; Ht, ante- rior three chambers of the heart, which usually extends to the posterior end of the body; Md, muscles of dia- phragm, the fibers spreading from the body wall to the heart; Ost, ostium, or oneofthe lateral openings into the heart chambers
in the dorsal part of the body; and by this means the food, now dissolved ira the body liquid, is carried into the spaces between the various organs, where the cells of the latter can have access to it. The heart of the insect is a slender tube suspended along the midline of the back close to the dorsal wall of the body (Fig. 67, 1/t). It has intake apertures along its sides I Vig. 69, Ost), and its anterior end opens into the body cavitv. It pulsates for- ward, by means of muscle fibers in its walls, thereby sucking the blood in through the lateral openings and discharging it by way of the front exit. An im- perfect circulation of the blood is thus established through the spaces between the organs of the body cavity, sufficient for the purposes of so small an animal as ara insect. The final act of nutrition comes now when the blood, charged with the nutrient materials ab- sorbed from the digested food in the alimentary canal, brings these materials into contact with the inner tissues. The tissue cells, by the inherent power of
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