A. D. Imms and M. a. Husain 277 



researches to examine as many as possible, recording the species which 

 respond, irrespective of whether they are injurious or otherwise. We 

 are of opinion that quantitative work of this kind will provide a broader 

 basis for a scientific understanding of the problems involved, than 

 specialised experiments conducted for the purpose of combating any 

 particular species of injurious insect. 



III. Methods. 



Experimental researches upon chemotropism divide themselves into 

 two groups — those conducted under the more or less artificial conditions 

 of an enclosed room, and those carried out under field conditions. It is 

 scarcely necessary to add that preliminary experiments in any branch 

 of physiology need to be carried out under definitely known and con- 

 trollable conditions of temperature, light, humidity, air currents, etc., 

 and in such a way as to eliminate as many of these factors as may be 

 desired. Such simplified conditions are scarcely possible in researches 

 of the nature under consideration, except in the case of experiments 

 conducted indoors. Our present researches are entirely concerned with 

 field experiments, and we are firmly of the opinion that preliminary 

 indoor trials are of little value in this connection. Insects reared and 

 liberated under artificial conditions, and surrounded by a totally alien 

 environment, do not necessarily react in a manner similar to those living 

 free in their natural habitat. In the case of insects concerned with the 

 spread of disease, and normally frequenting dwellings, hospitals, etc., 

 these remarks naturally do not apply, and their reactions form no part 

 of our observations. 



Chemotropic work in the field presents innumerable difficulties and 

 in no line of biological research with which we are acquainted is there so 

 large a number of environmental and other factors to be taken into 

 consideration. Success or failure of any chemotropic experiment depends 

 primarily upon the presence of favourable atmospheric conditions. One 

 is therefore largely at the mercy of climatic influences, which, in a 

 country like England, are extremely variable. Temperature, air-currents, 

 humidity, etc. have a direct bearing upon the evaporation of the sub- 

 stances to be tested, and the dissemination of their odours. Furthermore, 

 atmospheric changes influence the tropisms of insects to a marked degree. 

 Degree of temperature and amount of sunshine affect their activities, 

 a dull cloudy day may inhibit their movements very noticeably, a strong 

 wind interferes with flight, and excess of moisture inhibits their activities 

 in other directions. There are also many unknown factors which exercise 



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