Читать книгу Honey Bee Medicine for the Veterinary Practitioner - Группа авторов - Страница 72

In contrast to vertebrates, insects have an open compared to a closed circulation system with a single blood vessel and simple heart (Wigglesworth 1972; Ritter 2014). This system fills most of the body of insects outside of the other organs and tissues (Snodgrass 1956). Blood flow is produced by the action of a pulsating dorsal vessel that has a heart‐like function in the abdomen, directing the hemolymph forward through the aorta toward the brain; the blood returns via the body cavity to the abdomen to repeat the cycle (Wigglesworth 1972; Ritter 2014; Snodgrass 1956). Similar to vertebrates, hemolymph, or bee blood, contains many blood cells or hemocytes and functions to transport nutritional substances and waste products to and from the tissues, respectively; however, unlike vertebrates, insect hemolymph does not have hemoglobin and therefore cannot carry oxygen. In addition to their role in metabolism, insect hemocytes are also thought to have several other functions, including phagocytosis, resistance to microorganisms and parasites, blood coagulation and immunity (Wigglesworth 1972).

The vertebrate respiratory system is characterized by gills or lungs that act to exchange oxygen and carbon dioxide gases in concert with a complex circulatory system. A simpler method of gas exchange occurs in insects where the respiratory and circulatory systems are separate and blood has only a minor role in gas transport (Winston 1987). The respiratory system of most insects consists of a series finely branched trachea and tracheoles that allow for the direct diffusion of oxygen to their tissues and the removal of carbon dioxide (Wigglesworth 1972; Ritter 2014; Snodgrass 1956). Some insects, like the honey bee, also have saccular dilations of the trachea forming great air sacs. Air is brought into the insect through breathing apertures along the lateral thorax and abdomen known as spiracles (Snodgrass 1956). Ventilation of the air sacs and larger tracheal tree is thought to occur via specialized body wall movements that act to renew the tidal volume (Wigglesworth 1972). Most breathing insects have control over expiration alone, while inspiration is passive via the elasticity of the exoskeleton; however, honey bees have muscles that control both inspiration and expiration. The tracheal air sacs act largely as reservoirs for ventilation, and the respiratory movements of the abdomen in honey bees produce expansion and contraction of the air sacs similar to vertebrate lungs (Snodgrass 1956). Carbon dioxide is delivered via the hemolymph to the trachea for exhalation although elimination also occurs via direct diffusion through body tissues (Wigglesworth 1972; Ritter 2014).