i6 THE BIOLOGY OF INSECTS 



of the creature that is truly alive, is known as protoplasm, 

 the *' physical basis of life " as Huxley long ago called it. 

 Protoplasm is a semi-fluid material composed of various 

 elaborate nitrogenous chemical compounds known as 

 proteins. These substances are of high complexity ; in 

 addition to carbon, hydrogen and oxygen, nitrogen is always 

 present as well as a small proportion of sulphur, and a protein 

 is now regarded as " formed by the condensation of a 

 number of molecules of various amino-acids " (W. M. 

 Bayliss, 1924), different combinations of these '* building 

 stones " being characteristic of different types of protein. 

 The component molecules of protoplasm are in the colloidal 

 state. To repair the waste which living protoplasm con- 

 stantly undergoes, food in the form of protein must be 

 obtained by the insect, and such food is found either in the 

 tissues of plants, or in those of other insects or various 

 animals which may serve as prey or *' hosts," or in waste 

 or decaying organic substances. 



As mentioned in the introductory chapter, energy is 

 constantly liberated in insects as in living creatures generally, 

 becoming evident in movement and radiation of heat. 

 The " fuel " needed to supply this energy is largely furnished 

 by non-nitrogenous food-substances : carbohydrates such 

 as starch and the sugars, and fats. The energy liberated in 

 the activity of living tissues is due to the breaking down of 

 chemically complex substances, either components or in- 

 clusions of the cell-protoplasm. The renewal or rebuilding 

 of these requires a constant supply of food material, which 

 must be brought to the cells in a dissolved and absorbable 

 condition. An insect or other animal is truly fed only as 

 the living substance of its cells is fed, and the seizing and 

 swallowing of food, which is what many unobservant 

 persons understand by " feeding," is really only ingestion, 

 the necessary prelude to the true feeding process. 



We may then suitably begin our study of the feeding of 

 insects by considering some of the methods by which they 

 procure and swallow or ingest their foodstuffs. A well- 

 known feature characteristic of animals generally is the 



