FEEDING AND BREATHING 37 



into the blood, and serve along with the carbohydrates as 

 *' fuel-food." In very many insects comparatively large 

 masses of fatty tissue lie around the food canal in the great 

 blood-space, or on either side of the heart in the pericardial 

 wall. Such a *' fat-body " may reasonably be regarded as a 

 reserve of lipoid food-material on which the organism can 

 draw for the needs of its life-activities. The fat-body of a 

 mature larva is often relatively enormous, in view of the 

 coming transformation. Besides fat-globules the cells 

 of an insect " fat-body " contain protein granules, that 

 serve as a store of nitrogenous food. It has been shown by 

 G. A. Koschevnikov (1900) and others, by means of experi- 

 ments on larvae provided with coloured foodstuffs, that the 

 store of fat may be elaborated from nitrogenous food as 

 well as from the sugar (carbohydrate) present in honey. 

 Thus while much of the material on which an insect feeds 

 goes rapidly to repair its constantly wasting living substance, 

 or to supply the energy needed for its activities, much is 

 stored in forms convenient for future utilisation. 



The transformations of energy that continually go on in 

 the insect's tissue depend therefore on chemical reactions 

 between the cells themselves. The contraction of muscle 

 fibres, and the more delicate and obscure physical changes 

 concerned in the production of the nerve-impulses to be 

 discussed in some detail in our next two chapters, are due 

 to the liberation of energy resulting from chemical dissocia- 

 tion. It has been seen how these cells are supplied by the 

 blood with the necessary materials for building protoplasm 

 and with a constant stream of readily available fuel. These 

 supplies themselves depend on the digestive processes 

 carried on in the food- canal and due to the action of the 

 digestive ferments or enzymes on the ingested food-sub- 

 stances. The formation of the digestive juices is the work 

 of the cells of the salivary and gastric glands. These cells 

 are, like all other cells of the body, dependent for their 

 " raw material " on the substances supplied to them by the 

 blood, whence each gland extracts constituents which it 

 needs, and elaborates them into its own appropriate secretion. 



