116 THE PHYSIOLOGY OF INSECT SENSES 



the thicker walled of the two lumina to the tip of the hair. In triply 

 innervated hairs as originally described in Phormia by Grabowski and 

 Dethier (1954) and Dethier (1955 a) the fibres from two neurons extend 

 to the tip of the hair while the third terminates on the base. The exis- 

 tence in Phormia of hairs equipped with four neurons was only recently 

 suggested by behavioural evidence (Dethier and Evans, 1961) and 

 confirmed electrophysiologically (Mellon and Evans, 1961) and by 

 electron-microscopy (Larsen, 1962). Hairs with three, four, and five 

 neurons have been found in Calliphora (Sturkow, 1960; Peters, 1961). 

 The only part of the hair that is sensitive to chemical stimulation is 

 the extreme tip, which in some hairs is prolonged into a terminal 

 papilla into which the dendrites extend (Dethier, 1955 a; Dethier and 

 Wolbarsht, 1956). In the stable fly Stomoxys calcitrans Adams (1961) 

 has found that the terminal processes of the dendrites extend through 

 a pore in the cuticle (PI. III). 



OLFACTION 

 Acuity 



Although the behaviour of insects in their natural environment 

 suggests that their olfactory sense may be extraordinarily sensitive 

 by human standards, attempts to measure acuity accurately have not 

 been outstandingly successful. With the exception of Schneider's work 

 with Bombyx mori and associated species, electrophysiological meas- 

 urements have not been made. All measurements of the performance 

 of olfactory receptors have been based upon behavioural thresholds. 



80 



- )^ 60 



o 



^ 40|- 



UJ 



20f- 



UJ 



o 

 cr 



UJ Q 

 Q_ UJ 



20- 



IxlO'S IxlO'^ IxlO' 



MOLAR CONCENTRATION 



Fig. 72. Change in behaviour with change in concentration of the odour of 

 iso-valeraldehyde. (From Dethier, 1954.) 



