Blood and Circulation

All the arthropods that are the subject of this book possess an open circulatory system (Jones 1977). That is, the blood moves for the most part over and around the tissues and organs, bathing them and exchanging molecules with them directly, in a continuous body cavity, the hemocoel. In insects, there are no blood vessels save the main aorta that leads anteriorly, directly from the heart (McCann 1970), and empties into sinuses surrounding the brain. In centipedes, there are short lateral arteries leading to the gut and other minor vessels. There is a "pulmonary artery" to the book lungs in spiders as well as secondary vessels to the legs, tail, and so on, in other arachnids. The heart, which lies dorsally in the hemocoel, just beneath the abdominal roof, propels the blood forward with peristaltic contractions. After passing through the body cavity, including the legs, antennae, wings, and other appendages, and often aided by auxiliary, pulsatile organs at their bases, the blood reenters the heart through lateral pores (ostia).

The blood itself (or hemolymph) consists of a fluid plasma (Florkin and Jeuniaux 1974) in which nucleated cells are suspended (Crossley 1975). The latter are of many types but normally do not possess hemoglobin like vertebrate corpuscles, the oxygen/carbon dioxide transport function being assumed by the tracheal system (see below). Insect blood is not red (except in a few specialized types that have hemoglobin, such as blood worms, Chironomus) but green, a mixture of carotenoid and bile pigment ("insectoverdin"), bluish, or almost clear. The functions of the blood cells include phagocytosis, wound healing, coagulation, storage, and regulation of intermediate metabolism. The plasma serves primarily as the carrier of substances to tissues and also provides a store of nutritive compounds such as sugars and proteins. Its water acts as a reservoir for the maintenance of cellular fluids.

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