ELECTROPHYSIOLOGICAL INVESTIGATION 99 



number. This number must of course be great enough to ensure excitation in 

 the receptor loci. 



While a concentration of lO'^Vg ^^ filter paper could possibly still elicit a 

 response of the whole animal or its sense organs, concentration of less than 

 10"i2^/ginpositive behavioural tests (Butenandt etal., 1959, 1960, 1961, and 

 Fig. 5) appear to be " dangerously " low. To understand this problem it 

 is necessary to recognize that the only way to prepare descending concen- 

 trations is to dilute in the usual manner. Thus, all the figures given in 

 //g/ml solvent or in /(g on filter paper rely on the simple method of diluting 

 stock solutions. To date we do not have a better way or, indeed, a method 

 to check the actual amount of substance in the solvent. Hecker (un- 

 published) thought that an assembling of attractant molecules in the surface 

 layer of the solvent may occur. If so, the amount of molecules attached to 

 a glass rod would be higher than calculated. However, this explanation 

 did not receive experimental support, because tests done with lure substance 

 diluted in different solvents always gave the same very low threshold for 

 Bombykol. 



Finally, the potency of female glands was compared electrophysiologic- 

 ally with known amounts of lure substances. The normal glands of virgin 

 Bombyx females are as effective as a piece of filter paper holding between 

 10' Vg and 10//g of Bombykol. The amount of Bombykol that the chemists 

 were able to isolate per female was about 10"-//g. The chemically deter- 

 mined amount of ^-isomer per female gland for Porthetria, 5xlO"Vg^ was 

 also found to be in excellent agreement when checked experimentally. 



While these results are indicative, it is necessary to point out that the 

 paper piece or glass rod contaminated with the lure substance are only 

 finite reservoirs, eventually depleted by evaporation or oxidation. The 

 intact female gland, however, is a dynamic living system. It is a " factory ", 

 producing the lure substance in certain cells and then allowing the odour 

 to evaporate. Since the chemists reported that saponification of a certain 

 fraction of the extracted material increased the activity, it is probable that 

 part of the lure alcohol is stored in the gland in esterified form. However, 

 this is really all we can say. For a number of reasons it is neither possible 

 to estimate the number of molecules leaving the surface of the gland per 

 unit time nor the amount of substance present in the gland at a certain 

 moment. Consequently, the amount of lure substance a female produces 

 over its life span of a few days remains unknown. 



Biologically active substances transmitting information between in- 

 dividuals of the same species are called '' pheromones'' (Karlson and 

 Liischer, 1959 ; Karlson, 1960). Clearly the sexual attracting substances 

 belong to this class. The pheromonereceptors may be compared analo- 

 gously with a group of very intensively studied " chemoreceptors ", the 

 synapses. Here for example a chohnergic pre-fibre (Fig. 10) produces 



