36 THE BIOLOGY OF INSECTS 



heart through the paired slits in its wall, and thus the course 

 of the circulation is complete. 



In insects, as in the whole great group of the Arthropoda, 

 the blood, streaming through the spacious cavities of the 

 body, comes into direct contact with the tissues. Thus the 

 absorbed products of digestion can diffuse through the 

 wall of the stomach and intestine, so as to pass in solution 

 into the blood. And as the blood directly bathes all the 

 living tissues of the body it supplies continually to the celle 

 whether of muscle or nerve, gland or skin, germ or tube- 

 wall — the materials w^hich they need for building up the 

 complex proteins that compose their living protoplasm, 

 and the fuel that is required for the support of combustion 

 and the Hberation of energy. There is reason to believe, 

 from, analog}^ with what is now known to occur in verte- 

 brates (Bayliss, 1924) that the proteins of the cell are built 

 up of the amino-acids carried in the blood, so that in the 

 economy of the living body the analytic action of the digestive 

 organs is succeeded by a synthesis of protein. 



The carbohydrate " fuel-food " is absorbed by the 

 intestinal wall, circulates in the blood, and is suppHed to 

 the tissues in the form of monosaccharide. Insects have no 

 special organ comparable to the liver of vertebrates and 

 molluscs in v/hich the monosaccharide sugar is transformed 

 into polysaccharide glycogen and so stored up, to be drawn 

 on as required and reconverted into sugar circulating in the 

 blood. But in many insects glycogen probably forms a 

 carbohydrate store, and this may especially be expected to 

 occur in larvae preparing for the great expenditure of energy 

 involved in transformation. J. Straus has shown (191 1) 

 that nearly a third by weight of the substance of bee-grubs 

 after dessication consists of glycogen. Stored in the cells 

 of the fat-body, the glycogen is utilised for the reconstruc- 

 tion of tissue that marks the pupal period during which it 

 is retransformed into sugar and disappears. 



Fat is absorbed in the form of an emulsion or dissociated 

 into its component acid and glycerol, to be afterwards 

 re-synthetised so that the minute fat-globules are passed 



