CHEMICAL STIMULI 123 



Heliotropism or response to the stimulus of light, for example, 

 is frequently correlated with differential sensitiveness to variations 

 of luminous intensity. This results in a less direct form of 

 orientation towards, or away from, the stimulus, the orientation 

 being the resultant reflex response to a combination of heliotropism 

 and differential sensitivity. Thus, Mclndoo (1929) found that larvae 

 of the Codling Moth are weakly positively heliotropic, but they 

 were not found to move and orient themselves with the precision 

 implied by the theory of tropisms. They pursued sinuous paths 

 towards the light, and larvae in the second to fourth instars proved 

 to be positive to weak light and indifferent to strong light. When 

 about to spin their cocoons they appeared to lose all differential 

 sensitiveness and became strongly photonegative. More defined 

 examples of differential sensitivity are afforded by the behaviour 

 of many butterflies which are strongly positive to strong sunlight. 

 They become inactive immediately the sun becomes obscured, 

 and, when resting, some species direct the long axis of the body so 

 that the head is turned away from the source of light. Certain 

 species, when alarmed, no longer maintain their flight in the full 

 sunlight, but dart away and seek concealment in shade. In such 

 examples differential sensitiveness to light is a predominant feature. 



It is a matter of common observation that most insects are 

 active either only by day or only by night. We are led to the con- 

 clusion that this kind of behaviour is activated, not by any 

 tropism, but by a periodic rhythm dependent upon some internal 

 factor operating through the nervous system. This rhythm is 

 correlated with the regular diurnal alternation of daylight and 

 darkness, and cannot be explained as being solely due to the 

 stimulus of variations of light intensity. Many nocturnal insects 

 will remain quiet and motionless in a dark room throughout the 

 day and, without any change in illumination of the chamber, will 

 become active at a regular time each night. 



3. Chemical Stimuli and their Receptor Organs 



The reception of chemical stimuli is believed to take place in 

 insects by means of a variety of types of sensillae, whose functions 

 are largely deduced from their structure and their location. 



