A. D. Imm8 and M. a. Husain 271 



II. General and Historical. 



Cliemotiopisiii is the response of an organism to the stimulus of 

 chemical substances manifested, if the stimulus be responded to, in 

 movements towards (positive chemotropism) or away (negative chemo- 

 tropism) from the source of stimulus. Dewitz (1912) was the first to 

 emphasise the importance of the study of tropisms in relation to applied 

 entomology and dealt largely with phototropism. Tragardh (1913) laid 

 stress upon chemotropism and pointed out possibilities of its application 

 in combating destructive insects. There is no doubt that various tropisms 

 play a vital part in the economy of insect life but, of these, chemotropism 

 is the great controlling factor. The phenomenon is particularly evident 

 in the search for food, in the pursuit of the sexes and the selection of 

 suitable breeding places : it is also evident in many apparently unpurpose- 

 ful responses. As Loeb remarks (1918), the Aristotelian point of view 

 still prevails in biology namely, that an animal only moves for a purpose, 

 either in connection with those functions just enumerated, or in relation 

 to something else connected with the preservation of the individual or 

 the race. In the words of this writer, "Science began when Galileo 

 overthrew the Aristotelian mode of thought and introduced the method 

 of quantitative experiments which leads to mathematical laws free 

 from the metaphysical conception of purpose. The analysis of animal 

 conduct only becomes scientific in so far as it drops the question of 

 purpose and reduces the reactions of animals to quantitative laws." 

 While admitting the justice of Loeb's remarks, we do not consider it 

 desirable to eliminate all question of purpose from work of this natvire, 

 as the habits of a particular species not infrequently suggest the applica- 

 tion of substances of chemotropic value, improbable to come under notice, 

 except fortuitously, in other ways. Nevertheless, in many instances, it 

 is remarkabl}^ difficult to prove that the observed facts bear the relation 

 to a particular function which at first sight appears evident. The use of 

 baits to attract insects was known to agriculturists of a hundred years 

 ago, and w^hen our forefathers found their land to be infested by wire 

 worms they adopted the plan of burying beneath the soil slices of potatoes 

 impaled on skewers (Weiss, 1912). These were examined frequently and 

 the larvae which were attracted were collected and destroyed. One of 

 the -first important contributions to the scientific study of chemotropism, 

 with reference to insects, is the work of Barrows (1907) who investigated 

 the reactions of the pomace fly to odorous substances. This insect occurs 

 in great numbers around cider-presses, packing sheds, orchards and other 



