ELECTROPHYSIOLOGICAL INVESTIGATION 87 



areas or organs as in vertebrates (cf. also Dethier and Chadwick, 1948). 

 2. Which type of sensiUum is responsible for olfactory reactions? — The 

 answer to this question was gradually approached by earlier observations 

 which locahzed the main group of olfactory receptors on the antenna 

 (v. Frisch, 1919). But only in a few exceptionally fortunate cases was it 

 possible to identify the olfactory receptor proper. Notable examples, 

 accomplished by coating or amputation of sensilla, are the experiments of 

 Dethier (1941) and Bolwig (1946) with larvae of lepidoptera and diptera, 

 respectively, and of Wigglesworth (1941) with the human louse. In these 

 cases the sensilla basiconica were found sufficiently separated to be made 

 individually inoperative. The results showed that olfactory reactions of the 

 animals were not elicited after selective elimination of these sensilla. Most 

 of the other reported cases are still uncertain because of the presence of 

 many different types of sensilla packed closely together. Here, no proof 

 for an olfactory function of a specific sensiUum could be obtained with 

 classical methods. Very recently, however, Morita and Yamashita (1961) 

 using improved electrophysiological techniques on Bombyx larvae as well 

 as Boeckh (1962) and Schneider and Boeckh (1962) working with beetles 

 and moths, presented direct evidence for the olfactory function of sensilla 

 basiconica (Fig. 2). 



ELECTROPHYSIOLOGICAL STUDIES OF 

 INSECT OLFACTORY RECEPTORS 



The first successful investigations of this kind recorded multi-unit bursts 

 of nerve impulses from the antennae of cockroaches, bees and flies (Boistel, 

 1953, 1960 ; Roys, 1954 ; Smyth and Roys, 1955 ; Boistel, Lecompte and 

 Coraboeuf, 1956 ; Hodgson, 1958). The unknown receptors responded to 

 comparatively strong olfactory stimuli with increased impulse frequencies. 



In our laboratory we also tried to record the responses of highly 

 specialized olfactory receptors serving in the sexual behaviour of the silk- 

 worm and other moths. In these cases it was not only possible to record 

 the frequency modulation of olfactory nerve impulses but also a slow 

 olfactory potential (Fig. 3) called the electroantennogram or EAG 

 (Schneider, 1955; Schneider and Hecker, 1956; Schneider, 1957a and b; 

 Schneider, 1962). 



We suggested that the EAG is essentially the sum of many olfactory 

 receptor potentials recorded more or less simultaneously by an electrode 

 located in the sensory epithelium. This assumption has now received 

 experimental support, since we know that the single unit receptor potential 

 appears identical (cf. Fig. 2 with Fig. 3B and C). With extracellular 

 recording, the unit receptor potential and the EAG usually show the 

 expected polarity. The receptor site temporarily becomes negative, in 

 relation to the reference point. 



