SENSATION AND REACTION 75 



as the organs whence start those nerve-impulses which, on 

 arrival in the brain, result in sensations of smell or taste. 



In many insects sense-organs of this type are found in 

 large numbers on the feelers, and any increase in the com- 

 plexity of those appendages renders possible an increase 

 in the number of such olfactory nerve-endings. For 

 example, in the male moths mentioned above, that can be 

 attracted from a distance towards an imprisoned and con- 

 cealed female, the segments of the feelers are drawn out into 

 relatively elongate processes in which the special nerve- 

 endings are present in large numbers. The number and 

 arrangement of these organs enable an insect possessing 

 them not only to appreciate the vicinity of a female of its 

 own kind, but also to detect the Hne of advance along which 

 she may be found. The sense associated with nerve- 

 impulses starting from these organs is clearly similar to 

 our own sense of smell, while the large number of the organs, 

 their arrangement on the antennal processes, and the 

 mobility of the feelers all combine to give distinct indication 

 as to the direction of the source whence comes the odorous 

 vapour. Many beetles have the segments of the terminal 

 region of the feeler thickened or greatly expanded and 

 flattened, so that the feeler becomes clubbed (clavate) or 

 plate-like (lamellate) in form. The surface provided by 

 the enlargement of these segments are the seat of large 

 numbers of sense-organs of this olfactory type. The males 

 of such beetles often have antennal structures more elaborate 

 than those of the females, and their highly developed sense 

 of smell facilitates their detection of possible mates. But in 

 both sexes of these insects the olfactory sense serves as a 

 guide to their food which consists commonly of strongly 

 smelling material such as dung or decaying flesh, on which 

 substances the females also lay their eggs. 



Similar sensor}^ pores are also found abundantly on the 

 mouth parts of various insects, such as the maxillary palps, 

 the epipharynx, the tip of the labium, or in some cases the 

 mandibles. These have usually been regarded as organs of 

 taste, as by F. Will (1885), A. Forel (1908), and other 



