16 AN INTRODUCTION TO ZOOLOGY 



workers, which are sterile females and take no part in repro- 

 duction; queens, or sexually mature females; and drones, or 

 males. New individuals are formed by the fusion of reproduc- 

 tive or germ cells. The drone injects the sperm cells into the 

 queen, where they are stored up to unite with the egg cells as 

 they ripen within her body. 



(7) Irritability or reactiveness is a characteristic property of 

 organisms. It gives them the power to respond to changes in 

 their environment in such a way that they usually attain favorable 

 conditions for existence. Any change in conditions that pro- 

 duces the reactions of an animal is termed a stimulus. Stimuli 

 may be external or internal; the former includes changes in 

 temperature or light, in the composition of the surrounding me- 

 dium, etc. ; hunger and fatigue may serve as examples of internal 

 stimuli. 



In many instances animals show very uniform responses to 

 stimuli. An excellent illustration of this fact is furnished by the 

 katydid. The relation between the temperature and the number 

 of calls per minute is so constant in this insect that the exact 

 atmospheric temperature may be computed from the following 

 formula. 



, no. of calls per minute IQ 

 Temperature = 60 + - 



o 



The adaptive power of animals may be further illustrated by 

 the kidneys of man. In man, as in all vertebrates, these are 

 paired organs occupying a position near the dorsal side of the 

 body wall. Under normal conditions both eliminate an approxi- 

 mately equal amount of excretory matter. However, no perma- 

 nent injury necessarily results if one is removed, for the kidney on 

 the uninjured side of the body enlarges to such an extent that 

 it is able to carry on the functions previously performed by both. 



Instances of this adaptive power might easily be multiplied, for 

 they are common everywhere. In a broad sense, even the in- 

 telligence of man and the degree of success he may attain depend 

 upon how successfully his reactions meet surrounding conditions. 



