124 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



Chemical receptors are those of smell and taste ; while smell is 

 the perception of such stimuli acting from a distance, with taste 

 the distance factor hardly enters into account. It is probable 

 that among insects the two kinds of sensation are often not 

 sharply demarcated and that such a distinction may not be 

 necessarily valid. The antennae and palpi, for example, are used 

 to " explore " volatile substances, both at a distance and by 

 actual contact — a fact that suggests that their sensillae may 

 function both as distance (smell) and contact (taste) receptors. 

 If this be true the sensations appreciated may be merely those 

 of degree rather than different impressions. As Wheeler has 

 observed, the words " taste " and " smell " are charged with 

 anthropomorphism, and as the stimuli in both cases are chemical 

 in nature, it is better to refer to the receptors involved simply as 

 chemoreceptors. While admitting the force of this argument, it 

 appears desirable to recognise, at any rate tentatively, a distinction 

 between olfactory and gustatory organs. It must be remembered 

 that we have no evidence that sensilhc located within, or in close 

 association with, the oral cavity can function otherwise than as 

 gustatory or chemical contact receptors. 



The Olfactory Sense. It has long been known that insects are 

 highly scnsiti\'e to volatile substances, but the results of many 

 different experimenters are not in complete accord with regard 

 to the location of the olfactory sensillae. Various types of sensillae 

 occur distributed over the insect body and its appendages which 

 may conceivably function as olfactory receptors, but since two or 

 more types may occur grouped together, it often becomes a 

 difiicult matter for the experimenter to determine which particular 

 type may be concerned with the sense in question. It is easy to 

 prove that a given insect is definitely attracted or repelled by 

 certain volatile compounds, and there is now an extensive 

 literature on the purely empirical aspect of this subject. 

 Frequently it will be found that the olfactory sense is developed 

 more or less in inverse proportion to that of vision. Odonata and 

 Ephemeroptera, with their highly developed compound eyes and 

 dorsal ocelli, have their antennae reduced to small setiform 

 appendages. Chironomidae and Paussidae, with their highly 



