144 THE PHYSIOLOGY OF INSECT SENSES 



(Wolbarsht, 1957), that fructose stimulates L and S (Hodgson, 1957; 

 Wolbarsht, 1957; Evans, 1958), that sucrose stimulates two fibres in 

 Calliphora vomitoria (Morita, 1959), that bending stimulates L and S 

 (Hodgson et a!. 1955; but see also Hodgson and Barton Browne, 

 1960), that the primary receptor cell is responsive to chemical, tactile, 

 and thermal stimuli within the normal physiological range and hence 

 is at variance with the usual concept of single specificities of receptor 

 cells (Hodgson and Roeder, 1956; Hodgson, 1958 a, 1958 b). 



Another variable that must be considered is the possible differences 

 among hairs. Some hairs clearly respond more vigorously to bending 

 or to water than others. Some hairs in the female fly show discharge of 

 one fibre when protein such as crystalline haemoglobin or brain-heart 

 extract is applied, but in other hairs all fibres are silent when protein is 

 appHed (Dethier, 1961). In Vanessa also all hairs are not equally 

 sensitive to all compounds. 



In the course of recording from chemoreceptive hairs when mixed 

 stimuli were appHed, all workers have observed an interaction between 

 activity in the L and S fibres (Hodgson, 1956a, 1957; Morita et al. 

 1957; Wolbarsht, 1958; Morita and Takeda, 1959; Morita, 1959; 

 Sturckow, 1959; Takeda, 1961). Hodgson (1957) found that the 

 presence of 5* impulses is characteristically accompanied by a decrease 

 in L impulses and conversely that the stimulation of the L fibre is 

 associated with a decrease of S spikes. Since the inverse relationship 

 between the frequencies of S and L impulses is not constant, it does 

 not appear that the facts can be explained simply as a result of partial 

 depolarization of one receptor unit by electrotonic spread from the 

 more active adjacent unit. Wolbarsht (1958) also noticed this situation 

 and decided that the activity of the L fibre is not related to the activity 

 of the S fibre but depends only on the character of the stimulating 

 solution. He believed that such properties of the salt as thermo- 

 dynamic activity and the diffusivity of the salt can account in part for 

 the change in activity reported by Hodgson. On the other hand, 

 Takeda (1961) has concluded that the depression of the frequency of 

 impulses from the sugar receptor of Vanessa in the presence of NaCl 

 represents a direct inhibitory effect. 



At the present time the picture in the blowfly appears to be as 

 follows: one neuron is specifically sensitive to sugars; one neuron is 

 specifically sensitive to monovalent salts; the neuron whose process 

 terminates at the base of the hair is sensitive to mechanical stimula- 

 tion ; a fourth neuron, whose exact location has not yet been established 

 histologically, has as its adequate stimulus water. 



