REACTIONS TO CHEMICAL STIMULI 155 



inadequately understood does not advance knowledge of the 

 subject unless backed by proper experimental evidence. 



7. Reflex Behaviour and Practical Entomology 



The aim of the practical entomologist is insect control, and in 

 seeking to attain this object his first step is the investigation of 

 life-histories. If this study has been more or less thorough a 

 certain amount of data will probably have been collected with 

 respect to the behaviour of the species concerned. In the haste 

 to achieve practical results, further investigation of this kind will 

 probably be discarded, and the serious work of control embarked 

 upon. Unsatisfactory results so often follow this procedure that 

 it is coming to be realised that more basic studies are required, 

 and an increasing tendency is being shown to explore the wide 

 field of insect behaviour. It has come about that the sensory 

 reactions of insects now often form part of the programme in the 

 search for more adequate means of repression. The study of these 

 reactions is fundamental to many problems of applied entomology, 

 in that it affords possibilities that advantage may be taken of 

 specific kinds of behaviour in devising methods of control. 



Reactions to Chemical Stimuli. The most promising field in the 

 practical study of insect behaviour is afforded by their reactions 

 to chemical stimuli, which have come to be included under the 

 general term of chemotropism. Since the first critical experiments 

 were conducted in this subject by Barrows in 1907, a very con- 

 siderable literature has accumulated. It will serve, however, no 

 useful purpose to review all these numerous publications, and for 

 bibliographical information reference may be made to papers by 

 Imms and Husain (1920) and by Mclndoo (1927). Almost all the 

 investigations so far conducted in this field are of a purely empirical 

 kind, and have been prompted by n^ed for discovering chemical 

 compounds attractive or, less often, repellent to specific insects. 

 The Aristotelian view, that an animal only moves for a purpose, 

 has been the guiding principle in investigations of this kind. The 

 habits of a particular species, for example, frequently suggest the 

 practical ultilisation of chemical compounds improbable to come 

 under notice, except fortuitously, in other ways. Alongside work 



