THE CIRCULATORY SYSTEM 37 



storage of reserves of fat, glycogen and protein (p. 68) and sometimes 

 of uric acid and lime (p. 56). But it has also important functions in 

 intermediary metabolism. The cells are stufTed with mitochondria. 

 They are almost as rich in diverse enzymes as the mammalian liver: 

 esterase (lipase) to liberate fatty acids from the stores of fat; succin- 

 oxidase and all the other enzymes of the citric acid cycle which 

 furnish hydrogen as fuel for the cytochrome system; transaminases 

 which convert one amino acid to another; enzymes to convert glu- 

 cose to trehalose; enzymes concerned in diamination, uric acid syn- 

 thesis, purine oxidation and many more. They contain much ribo- 

 nucleic acid, and synthesize protein for the circulating blood and for 

 supply to the developing eggs. In some insects, notably in the larva 

 of Gasterophilus, certain fat body cells (so-called 'tracheal cells') be- 

 come laden with haemoglobin; in the aquatic bugs Buenoa and 

 Anisops the haemoglobin in similar cells serves as a store of oxygen 

 during diving. 



(iii) Unlike the other tissues considered in this chapter, all of 

 which are of mesodermal origin, the oenocytes are derived from the 

 ectoderm. They often arise throughout the larval stages, for example 

 in Rhodnius, from the ordinary epidermal cells. They may remain 

 between the epidermis and basement membrane, or they may become 

 distributed throughout the fat body. These cells also are probably 

 important in intermediary metabolism. The only suggested function 

 for them for which there is positive evidence is that they are ecto- 

 dermal cells which have become specialized for the production of 

 some particular constituent of the cuticle (and perhaps of the egg- 

 shell) such as the lipo-protein of the cuticular layer or the cuticular 

 wax (p. 4). During the early stages of cuticle formation droplets of 

 lipid-containing substance can be seen to pass from the oenocytes to 

 the epidermal cells for deposition over their outer surface to form the 

 'cuticulin' layer of the epicuticle. 



BIBLIOGRAPHY 



florkin, m. and jeuniaux, c. Physiology oflnsecta, III (Morris Rockstein, 

 Ed.), Academic Press, New York, 1964, 110-152 (composition ot 

 the haemolymph) 



