356 



LL. LLOYD. 



less so, but a weak solution of the same substance was the most attractive in the 

 test, trapping nearly a quarter of the total flies caught. 



It was evident from these and from other crude tests that the attractive solutions 

 all had a predominating yellow colour, or were fluorescent. The next experiments 

 were carried out in an endeavour to find an optimum attraction. 



Experiment 9. 



December 9th-15th ; httle weak sunhght ; exposure 56 hours' dajdight. Solutions 

 of potassium bichromate (yellow fluorescent) of percentage strengths 5, 1, J, -■^^, 

 -^--^ _i_, two tubes of each ; 1,771 flies were trapped, and their distribution- is shown 

 in diagram 2. The optimum strength for this solution was clearly from I to -^^ per 

 cent., from the nature of the curve between these two ; i per cent, potassium 

 bichromate was therefore used as a standard in the next experiment. 



Diagram 1. (See Experiments 5_and 6.) 



Experiment 10. 



December 16th-20th ; little weak sunlight ; exposure 40 hours of daylight. 

 Four solutions of orange G. of strengths J, -^^. lis, and ^f^ per cent., and four of eosin 

 of strengths I. ^-^, Jj, and ^ie per cent., one tube of each, were tested against four 

 control tubes of ^ per cent, potassium bichromate ; 983 flies were trapped, and had 

 the distribution shown in diagram 3. The eosin solutions were relatively unattractive, 

 but the nature of the curve shows that there was some attraction, the optimum 

 strength being about ji^ per cent. It may be pointed out that the occasion when 

 the attraction of this substance was powerful was when tested in bright sunlight, 

 its other tests being all in dull weather with intermittent weak sun. The solutions 

 of orange G. were less attractive than the control except the ^Is per cent, solution, 

 which drew 60 per cent, more flies. In the course of the experiment three counts 

 were made on the twelve tubes, and this tube trapped most flies in each case, except 

 that it was bettered once by a control, 53 as against 42. An attempt was made to 

 analyse the maximum point on the orange G. curve in the next experiment. 



