CHEMORECEPTION 117 



As the concentration of a stimulating odour exceeds that necessary 

 to stimulate the most sensitive receptor element, a behavioural res- 

 ponse occurs in the form of antennal movements, movements of the 

 mouth-parts, occasionally salivation or grooming movements whereby 

 legs or antennae are drawn through the mouth-parts or rubbed against 

 one another. A further increase in concentration results in an oriented 

 movement which may carry the insect towards or away from the 

 source of odour. If the initial response is towards the odour, an addi- 

 tional increase in concentration may cause a reversal in orientation 

 (Fig. 72). An odour which is initially repellent usually remains so at all 

 higher concentrations. An attractive odour may become repellent at 

 higher concentrations (Dethier, 1947 b; Dethier, Hackley, and Wag- 

 ner- Jauregg, 1952). 



Measurements of acuity thus depend in large measure upon which 

 behavioural threshold is chosen for testing. Numerous attempts to 

 measure threshold have been made (Barrows, 1907; von Frisch, 1919; 

 Wirth, 1928; Folsom, 1931; Warnke, 1931; Reed, 1938; Wietingand 



Table 1 



(From Schwarz, 1955) 



Differences between the olfactory thresholds of man and honeybees 



