The principal function of the blood, or hemolymph, is the transport of materials-nutrients, hormones, wastes, and so forth. In most cases it plays a relatively minor role in the transport of oxygen and carbon dioxide. The hemolymph is also involved in osmoregulation, the balance of salts and water in the body; this function also involves other organs, particularly the Malpighian tubules and the rectum. The hemolymph has an important skeletal function-for example, in molting, in the expansion of the wings after the last molt, and in the protrusion of eversible structures such as eversible vesicles and genitalia. It may also function in the animal's internal defenses, in the phagocytic action of hemocytes against invading microorganisms, in plugging wounds, and in walling off certain foreign bodies such as endoparasites. Finally, the hemolymph is also a storage tissue, serving as a reservoir for water and such food materials as fat and carbohydrates.

The circulatory system of an insect is open. The main (and often only) blood vessel is located dorsal to the alimentary tract and extends through the thorax and abdomen (Figure 1). Elsewhere the hemolymph flows unrestricted through the body cavity (the hemocoel). The posterior part of the dorsal blood vessel, which is divided by valves into a series of chambers, is the heart (hr), and the slender anterior part is the aorta (ao). Extending from the lower surface of the heart to the lateral portions of the terga are pairs of sheetlike muscle bands. These constitute a dorsal diaphragm more or less separating the region around the heart (the pericardia! sinus) from the main body cavity (or perivisceral sinus, sometimes further divided into a perivisceral sinus and a perineural sinus). The heart is provided with paired lateral openings called ostia, one pair per heart chamber, through which hemolymph enters the heart. The number of ostia varies in different insects. Some have as few as two pairs.

Fig1. Internal organs of a grasshopper, shown in longitudinal section (somewhat diagrammatic). ans, anus; ao, dorsal aorta; be, bursa copulatrix; ca, corpus allatum; cec, circumesophageal connective; cg, cerebral ganglion (part of the brain); em, gastric caeca; en, colon; cp, crop; eg, eggs; eso, esophagus; gn, ganglia of ventral nerve cord; hr, heart; hyp, hypopharynx; il, ileum; lbm, labium; lbr, labrum; mg, midgut or mesenteron; mt, Malpighian tubules; nc, ventral nerve cord; og, optic ganglion (part of the brain); ovd, oviduct; ovp, ovipositor; ovt, ovarian tubules; phx, pharynx; rec, rectum; segn, subesopha geal ganglion; slg, salivary gland; sld, salivary duct; spth, spermatheca; vag, vagina. (Redrawn from Robert Matheson: Entomology for Introductory Courses, Second Edition. Comstock Publishing Company, Inc.)

The hemolymph is usually a more or less clear fluid in which are suspended a number of cells (the hemocytes). It may be yellowish or greenish but is only rarely red (as in some aquatic midge larvae and some aqu81tic Hemiptera, owing to the exceptional presence of hemoglobin). It makes up from 5% to 40% of the body weight (usually about 25% or less).

The liquid part of the hemolymph (the plasma) contains a great many dissolved substances (such as salts, sugars, proteins, and hormones). These vary considerably-in different insects and in the same in sect at different times. The plasma contains very little oxygen; the transport of oxygen is the function of the respiratory system and is decoupled from the circulatory system.

The hemocytes vary considerably in number from about 1000 to 100,000 per mm3-but average about 50,000 per mrn³.These cells vary greatly in shape and function. Some circulate with the hemolymph, and some adhere to the surface of tissues. The functions of the various types of hemocytes are not well known, but many are capable of phagocytosis. They may ingest bacteria, and they play a role in re moving dead cells and tissues during metamorphosis. The hemolymph of different insects differs in clotting ability; the hemocytes may migrate to wounds and form a plug. Hemocytes often congregate around foreign bodies such as parasitoids and parasites, forming a sheath around them and walling them off from the body tissues. Other than the action of these hemocytes, insects have no immune system comparable to the antibodies of vertebrates (thus facilitating tranplantation experiments).

Hemolymph is moved about by pulsations of the heart and in other parts of the body, such as the base of the legs and wings, by accessory pulsatile organs. The heartbeat is a peristaltic wave that starts at the posterior end of the dorsal blood vessel and moves forward. Hemolymph enters the heart through the ostia, which are closed during the systolic phase of the heartbeat, and is pumped anteriorly. The rate of the heartbeat varies greatly: Observed rates in different in sects range from 14 to about 160 beats per minute. This rate increases during periods of increased activity. The pulsations of the heart may be initiated within the heart muscle (myogenic), or they may be under nervous control (neurogenic). A reversal of the direction of the peristaltic wave of contractions, thus moving the hemolymph backward instead of forward, is not unusual.

Very little pressure develops in the general flow of hemolymph through the body. The hemolymph pressure may sometimes be less than atmospheric pressure. It can be increased by muscular contraction and com pression of the body wall or by dilation of the alimentary canal (produced by swallowing air). This is how pressure is developed to break out of the remainder of the old exoskeleton at molting time and to inflate the wings.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم صناعة الشبابيك يواصل أعمال الإدامة لأحد أبواب حرم مرقد أبي الفضل العباس (عليه السلام)

ضمن فعاليات ملتقى النورين.. المجمع العلمي يقيم ندوة فكرية في بابل

المجمع العلمي يحيي ليلة القدر الأولى في مقام الإمام المهدي (عجّل الله فرجه)

مركز السيد جعفر الحلي الطبي يقدم الاستشارات والخدمات الصحية في ليالي القدر المباركة

المزيد

الآخبار الصحية

خمسة مؤشرات صحية مهملة قد تنقذ حياتك

متى تكون الحمى طبيعية؟

التدخين الإلكتروني يهدد قلبك.. دراسة جديدة تكشف مخاطر ارتفاع ضغط الدم

دراسة تكشف فائدة بعض أنواع الشاي في محاربة الاكتئاب

المزيد

الآخبار التكنلوجية

الجهاز المناعي يظهر دورا غير متوقع في صحة العظام

ابتكار من جامعة الشارقة منخفض التكلفة للحماية من الزلازل

اكتشاف تأثير غير متوقع للكاكاو على وظائف الدماغ

باحثون يرصدون أثر تغيّر المناخ في دم البشر

المزيد

مواضيع ذات صلة

Excretory System of Insects

Digestive System of Insects

Epidemiology of Lyme disease

Protection and defence by the haemolymph in Insect

Hormones in insect

الانسلاخ Moulting or Ecdysis

Crawling, wriggling, swimming and walking

Cephalic sensory structures

الأمراض التي تنقلها الصراصير الحمراء

Colour production

رتبة البراغيث Siphonaptera

رتبة ثنائية الأجنحة (Diptera)