Sensory Discrimination: The Chemical Sense 109 



the solitary wasps by Mr. and Mrs. Peckham, it appears 

 that sight plays a far more important role than smell for 

 these insects, and the return to the nest in particular seems 

 to be almost entirely an affair of sight (572, 573). In gen- 

 eral, the greatest development of qualitative variety in the 

 sense of smell is found in the social Hymenoptera, and is 

 probably a product of the social state. Ferris, however, 

 noted that the solitary wasp Dinetus was much disturbed 

 in finding its nest hole if he had placed his hand over the 

 hole during the wasp's absence, and thought the odor of 

 his hand was distracting to the insect (574). 



30. The Chemical Sense in Vertebrates 



Although the vertebrates stand at the head of the animal 

 kingdom, yet in point of complexity of structure and behav- 

 ior the lowest vertebrate is far below the highest members 

 of the invertebrate division. When we undertake to study 

 the responses to special stimulation displayed by this 

 same lowest vertebrate, the little Amphioxus or lancelet, 

 it is like going back to the earthworm. The only kind of 

 evidence that contact, chemical, and temperature stimuli 

 produce specific sensation qualities is found in the fact that 

 sensibility to them is differently localized, and may be in- 

 dependently fatigued. To weak acid, the head end of the 

 animal is most sensitive, the posterior end less, the middle 

 least; to contact with a camePs-hair brush, the two ends 

 are equally sensitive and more so than the middle; to a 

 current of warm water the order of sensitiveness is : head 

 end, middle, posterior end (541). 



For fishes, as for all aquatic animals, the distinction be- 

 tween smell and taste becomes obscure. The neighborhood 

 of food not in actual contact with the body seems to stir 



