34 ELEMENTARY STUDIES IN INSECT LIFE 



CHAPTER II 

 THE SPECIAL SENSES 



Value of Sense Organs. --In animal life there arises 

 with the development of the nervous system, the need 

 of transmission of impressions from certain nerve cen- 

 ters to the muscles. This need is fulfilled by the means 

 of motor nerves. External impressions are to be con- 

 veyed inwardly, to special nerve centers. This function 

 is performed by the sensory nerves. In the simplest 

 forms of life, such as the Amoeba, the body of which 

 is composed of but a single cell, special sensory organs 

 are wanting. The outer part or surface is a general 

 sensory organ. While the sensory phenomena of these 

 primitive forms are not well known, it has been ascer- 

 tained that they are sensitive to external stimuli, such 

 as electricity, contact with other bodies, heat, and the 

 actions of certain chemicals. Many are sensitive to 

 light. As the scale of life advances, and animals be- 

 come more complex, the sensory areas become more 

 localized, and their functions more varied. When 

 sensory nerves become grouped in one locality or organ 

 of the body, for a special purpose, we call that location 

 or organ, with its nerves, a sense organ. With this 

 localization there comes also an increase in the power 



