32 INSECT PHYSIOLOGY 



maintain around the axons a tissue fluid with a very low potassium 

 content. 



Another striking feature is the high content of amino acids, which 

 may be twenty or thirty times as great as in human blood and form 

 nearly 15 per cent, of the total nitrogen. In addition, there is a con- 

 siderable amount of residual nitrogen in peptide form. Proteins may 

 be present in much the same amounts as in mammalian plasma 

 (around 6 per cent.) but usually there is much less than this. They 

 can be grouped as 'albumens' and 'globulins', but by the use of 

 electrophoresis and antigen - antibody precipitation as many as ten 

 proteins can often be distinguished. Many of these are conjugated 

 with lipids, sterols, or carbohydrates and most of them are enzymes 

 of one kind or another. 



In the honey-bee and some other insects the chief sugar of insect 

 blood is glucose, but in most insects there is almost no fermentable 

 sugar in the haemolymph; the chief carbohydrate is trehalose, a non- 

 reducing glucose-glucose disaccharide which may sometimes be 

 present at a concentration exceeding 5 per cent. A great variety of 

 organic metabolites (such as succinate, pyruvate, citrate, &c.) and 

 notably organic phosphates (such as a-glycerophosphate, glucoses- 

 phosphate, &c.) are present in insect haemolymph at levels that occur 

 in vertebrates only within the tissue cells. Whereas fermentable 

 sugars are often absent from the haemolymph, other reducing sub- 

 stances (perhaps phenols) are plentiful. 



Cytology of the blood 



The blood-cells or haemocytes of insects, as seen in preparations, 

 present an extraordinary diversity of appearance; but this seems to 

 be due to the protean forms which they can assume rather than to 

 the abundance of distinct types. They multiply and grow in the body 

 cavity throughout the life of the insect, appearing first as small 

 darkly staining forms termed 'proleucocytes', which are often seen 

 dividing and are not yet capable of phagocytosis. As they grow they 

 become pyriform or spindle-shaped and will ingest dead bacteria, 

 Indian ink, tissue debris, &c. ; they are then called 'plasmatocytes'. 

 Their appearance naturally varies very much with their content, and 

 sometimes separate names have been given to these phases; more- 



