94 DIETRICH SCHNEIDER 



mostly stays behind Bombykol. The lO-cis-, \2-trans isomer usually is 

 second in order, followed by \0-cis, \2-cis and then \0-trans, \2-trans. 

 Relatively strong isomer-responses (see lO-r/5, \2-cis in Fig. 6) appear fre- 

 quently at lower concentrations and cannot be adequately explained yet. 



One remarkable feature of the Bombykol reaction curve is the enormous 

 concentration range covered by the antenna as a whole (Fig. 6). Equally 

 wide ranges of about 10 ^^ ^re known only for the human eye and ear. 

 Even more striking is the Bombykol response curve. From threshold below 

 lO'^Vg to about lO'Vg. the reaction is practically constant. With 

 higher concentrations the intensity of the reaction first rises slowly and then 

 at a very fast rate. 



From this response of the antenna to Bombykol, it may be deduced that 

 the animal is not able to distinguish between different molecular densities 

 with great sensitivity in the lower concentration range. But density dis- 

 crimination apparently becomes excellent in the higher range. This re- 

 lationship now appears to fit ideally to the requirements of a male moth in 

 nature. Perhaps a male, several hundred meters downwind from a female, 

 just " catches " with its olfactory receptors the minimum number of lure 

 molecules necessary to elicit excitation. This stimulus works as a signal to 

 induce or reinforce a positive anemotaxis (Schwinck, 1954) and thus 

 eventually leads the male to the neighbourhood of the female. During this 

 approach, the olfactory receptors of the male will constantly give the same 

 low level information to the brain, keeping him moving upwind. Odour 

 concentration differences due to lack of laminarity in the air stream will 

 rarely be large enough to change the information output of the olfactory 

 receptors. This could be very important, since the male might otherwise 

 deviate from the upwind direction in pursuit of every meaningless concen- 

 tration increase of lure odour. Finally, rather suddenly when the odour 

 concentration indicates the neighbourhood of the quietly waiting female, 

 an olfactory orientation primarily due to intensity discrimination may be 

 accomplished. 



In support of this hypothesis is the following observation. Dissected 

 glands of females are electrophysiologically as effective as filter paper hold- 

 ing between 10"-//g and 10//g of Bombykol. This concentration range is 

 exactly where the gradient of the curve (Fig. 6) is steepest and where odour 

 intensity discrimination is expected to be optimal. The stimulus-response 

 function of the antenna stimulated with Bombykol thus appears to ad- 

 mirably fulfil those requirements which may best serve the male in the 

 field. 



Adaptation has not been extensively studied yet. However, it probably 

 is the main cause of the hysteresis curve obtained in an experiment where 

 the antenna was stimulated with ascending Bombykol concentrations first 

 and descending concentrations afterwards (Fig. 7). Reference stimuh of 



