THE FUNCTIONS OF INSECT BLOOD
Authors: MELLANBY KENNETH 
Published: January 2008

Insects have a haemocoele, in which the only tissue fluid, the blood, circulates. The blood consists of haematocytes and plasma.

Most of the haematocytes are phagocytic leucocytes. These alter considerably in appearance at different stages of deuelopment, assuming many different forms. Respiratory corpuscles are never present.

The phagocytes assist in breaking down obsolescent tissues. They are most active during the pupal period in endopterygote ikects, but also exhibit considerable activity at each moult in both endopterygote and exopterygote forms. Haematocytes take some part in producing internal membranes.

The phagocytes serve to protect the insect's body against invasion, particularly by bacteria. Immunity against bacterial diseases can be developed, either naturally or by processes analogous to vaccination, and in immune insects the phagocytes attack the bacteria more rapidly and more successfully than otherwise. Metman parasites may be overcome by the haematocytes, which then surround the invader with a capsule. Parasites which are adapted to living inside specific insect hosts suffer no inconvenience from the activities of the blood cells. The capsules produced may even be necessary for the well‐being of these parasites.

The plasma is a viscous fluid which may be coloured or colourless. Except for haemoglobin in solution in the plasma of chironomid larvae, no respiratory pigments are found. The fluid consists mainly of water, but the percentage of dry matter, the proportion of many of the various constituents and the reaction of the liquid shows considerable variations even in the same insect at different stages of development.

The water in the plasma serves as a useful reserve, and allows the insects to withstand considerable desiccation. Under such circumstances the blood becomes more concentrated and more viscous.

Food substances are transported and stored in the blood, which also carries hormones about the body.

Respiration in insects is primarily the function of the tracheal system, but the blood has certain subsidiary functions. The tracheoles do not enter all tissues, and some cells receive their oxygen from solution in the blood, which serves as an intermediary between the tissues and the tracheae. Various structures have been described as being especially adapted to oxygenate the blood, but these have mostly been found to be unimportant. The osmotic pressure of the blood may govern the extent to which air extends into the tracheoles. Muscular activity liberates metabolites which increase the blood's osmotic pressure, and this removes some fluid from the tracheoles and draws the air up among the active tissues, thus increasing their oxygen supply.

The blood sometimes contains poisonous substances; “reflex bleeding” may then protect the insect from attack.

The plasma as well as the haematocytes may be concerned in producing immunity to bacterial infection.

The blood has important mechanical functions. It is the means by which pressure is transferred from one part of the body to another, and thus assists in hatching and moulting. Desiccation, by reducing the blood volume, may interfere with these processes. The volume of blood also serves to maintain the body size, and if the volume is decreased during development an undersized adult may result.