REACTIONS TO CHEMICAL STIMULI 159 



one fork of a Y-tube by means of a suction apparatus. Experi- 

 ments carried out with the Colorado potato beetle {Leptinotarsa 

 decemlineata) showed that when individuals were introduced 

 into the Y-tube an average of 62-7 entered the arm through which 

 air from the chamber containing the potato plant was being drawn. 

 The insects also responded to the odours of water extracts of 

 potato tubers and potato foliage ; odour from the steam distillate 

 of the tubers was only slightly attractive, while a more pronounced 

 response was obtained in the case of distillates of the potato- 

 foliage. Power and Chesnut (1925), acting upon the assumption 

 that the cotton boll weevil is attracted to its food-plant by some 

 odorous volatile substance, undertook to isolate this substance. 

 Among the twelve substances isolated three appeared to be of some 

 significance, viz., an essential oil, ammonia and trimethylamine. 

 The percentage of oil isolated appeared to be too small to be of 

 chemotropic significance, and, despite the fact that more ammonia 

 than trimethylamine was found, they concluded that if the cotton 

 plant contains a positive chemotropic constituent it is probably 

 trimethylamine. No very conclusive results appear to have been 

 derived from this work, and Mclndoo doubts whether it is possible 

 to reproduce accurately the odour or odours, which emanate from 

 a plant, by using certain constituents obtained from the plant by 

 chemical means. In 1927 Richmond, experimenting with various 

 essential oils with reference to the discovery of agents attractive to 

 the Japanese beetle {Popillia japonica), found sassafras oil to be 

 by far the most efficient in this respect. The constituent of this 

 oil which exercised the most powerful attraction to the insect 

 proved to be geraniol, which is an unsaturated alcohol belonging 

 to the class of terpene compounds. This alcohol appears to be 

 one of the most powerful specific chemotropic agents so far 

 discovered, and enormous numbers of the Popillia have been 

 caught, through its agency, in suitably designed traps. Although 

 geraniol occurs as a constituent of the apple, which is one of the 

 favourite food-plants of the beetle, it is by no means clear as to 

 what part it plays as an agent influencing the selection of this 

 and other plant species by the insect in question. 



A considerable amount of investigation has been devoted to 



