CHAP. VI. | I R( »PISMS. 57 



paint, the insect flies or creeps in circles, with the unaffected eye 

 always turned towards the centre. 



It cannot be too stronglj emphasized that, given the requisite 

 conditions, these tropic reactions arc hound to take place ; they are 

 absolutely independent ol volition and many ol them can be 

 reproduced with purelj mineral or unorganized material, being 

 brought about by ordinary chemical ami physical forces. It follows, 

 therefore, thai the fuller investigation of these tropisms forms a verj 

 promising Held for research work in connection with the control ol 

 insect-pests ; for, it we can find methods by which any particular 

 insect may be attracted in obedience to a tropic reaction, we shall 

 be in possession of a very powerful weapon for its control. Such 

 attraction is usually strongly specific and often confined onlj to 

 one sex of a particular species, both these conditions being instanced 

 in the attraction of fruit flies to essential oils; here it is only tin 

 males which are attracted and. generally speaking, each species is 

 attracted by a different oil. In this case the attraction is of little 

 practical use as only the males are caught and these have generallj 

 fertilised the females before capture so that little actual control is 

 achieved. But in other cases, notably in that of Agrotis ypsilon, 

 greater success has been attained and that on a practical field- 

 scale whereby thousands of acres of crops have been saved which 

 in previous years had been damaged by the caterpillars of this 

 moth. It has been found, in this case, that the female moths 

 require to feed in order to develop their eggs successfullj b< 

 these are laid and that during this period, which lasts a week or 

 ten days, the females may be attracted by exposing a mixture of: 



Water ... ... ... 200 parts ii\ weight. 



Sugar or jaggerj ... ... 400 



Al< ohol ... ... ... 3 



Ethyl aci tate ... ; 



Trials of this mixture with the closely-allied Euxoa segetis in 



Southern India, however, have been unsuccessful, so that this 

 mixture also appears to be strongly specific. 



The use of light-traps for the capture of crop-pests has already 

 been mentioned as an obvious instance of the practical application 

 ol a tropii reaction. But here again we require more exact experi- 

 ments to ascertain what species of insects are attracted and in 

 what proportions, both of the total number of individuals in a given 

 area and of the relative proportions of the sexes, and (for each 

 insect) the optimum meteorological conditions under which light- 

 traps may be used, and the relative efficiency ot different values oi 

 light both as regards strength and colour. 



