50 



CAPT. LL. LLOYD. 



being taken of the numbers of flies which visited and tasted the solution and then 

 left, and of those which remained to drink. Controls were made with tap water. 

 A fly only " tastes " when it presses its proboscis one or more times against the wet 

 surface, and quickly withdraws it. It " drinks " when the proboscis remains pressed 

 against the surface for a few seconds or longer. Apart from the count, an impression 

 may also be gathered from the behaviour of the flies. On a fluid which they find 

 attractive they cluster, but on an unattractive one there are never more than two 

 or three flies at one time. The total number of visiting flies was about constant 

 in each test. 



The disadvantages of the presence of formic acid in formaldehyde are shown 

 by the following tests, where a solution containing 12 grms. per litre was tested 

 against the one containing 1'8 grms. per litre. In ten tests the less acid solution 

 killed twice as many flies as the more acid one. 



Table V. 

 Showing the Disadvantages of the presence of Formic Acid in Formaldehyde. 



Solution 



Percentages 

 flies killed 



of 





Av 



. per cent, 

 killed 



Total 

 killed 



Formaldehyde 

 »» 



1-8 grms. 

 HCO-OH 

 per lit. 10 Oq 



12-0 grms. 

 HCO-OH 

 per lit. 10% 



100 in each case 



54, 78, 54, 27, 34, 

 51,43,50 



38, 



34, 





100 



50-6 



808 

 366 



Flies are less readily repelled by dilute alkahne than by acid solutions, and will 

 drink solutions containing O'l to 0'2 per cent, caustic potash, or 0'07 per cent, 

 lime, as readily as water. There is possibly even a little attraction in it. The fol- 

 lowing Table VI shows this, the data being obtained in the same wav as those in 

 Table IV. 



Table VI. 

 Shoiving that certain weak Alkaline Solutions are not deterrent. 





No. of 



Av. flies 



Av. flies 



Av. percentage 



Solution 



tests 



tasting 



drinking 



which drank 



Tap- water control 



5 



38 



31 



83 



K0H2% 



1 



35 











„ 0-3-0-4% 



y 2 



31 



14 



45 



„ 0-1-0-2% 



2 



M 



30 



91 



Ca(0H)2, 0-14% (lime 











water) 



2 



37 



20 



54 



0-078% 



2 



33 



20 



61 



0-07% 



3 



37 



30 



81 



Temperature 77° Fr ; relative humidity 57%. 



