52 



CAPT. LL. LLOYD. 



it must with any poison which is administered as a drink. In wet weather flies- 

 have less inclination to drink and there is more moisture available, so that they 

 are less Ukely to discover the trap. Experiments were made to test this. 



(i). A jar with an area of 2*5 sq. in. was filled with sand wetted with a 10 per cent, 

 solution of formaldehyde without any addition. It was placed in the centre of a 

 dish of dry sand, area 95 sq. in., and this was placed in a greenhouse containing a 

 large number of flies. No water was available and the atmosphere was dry. After 

 twenty-four hours 1 ,560 dead flies Avere counted on the surface of the sand, or within 

 a few inches of the dish. The experiment was repeated with approximately the 

 same number of flies available and the same atmospheric conditions, but in this 

 case the sand in the surrounding dish was wetted with water, equally with the 

 poisoned surface. After twenty-six hours only 20 flies were found dead, the majority 

 having obtained their water from the unpoisoned surface. 



(ii). This series was carried out in a glass cylinder 24 in. high and 10 in. in diameter. 

 Wet and dry bulb thermometers were included. The formaldehyde (10 per cent. 

 in water) was supplied soaked up in sand, area 2 '5 sq. in. The duration of each 

 experiment was twenty- two hours. The air was in no case quite dry, as the amount 

 of moisture given of? by the poisoned surface is considerable in this confined space. 

 It was found that the formaldehyde was effective with a relative humidity of 76- 

 per cent, and 72 per cent., that its effect was about halved with a relative humidity 

 of 81 per cent., and that above 90 per cent, it was much reduced, available water 

 being present throughout in this case. The results are given in Table VIII. From 

 these it may be concluded that Hquid poisons will not be very effective if 

 much other available moisture, such as dew, is present. Formaldehyde works 

 well in a room where there is a wet sink, as was the case in the fly room where these 

 experiments were performed. It would be useless, however, to place the poison 

 in the sink or immediately over it, though it will be found very effective a few feet 

 away. Fhes are usually going about looking for food, and seem to taste all moisture 

 they come across in their wanderings, either in the hope of its containing food or 

 simply for the sake of the drink. Their capacity for utilising solid food depends 

 on whether the crop contains fluid with which to dissolve it. 



Table VIII. 



Showing the decreasing effect of Fonnaldehyde ivith increasing Humidity. 



Conditions in Cylinders 



Temp. 



1 

 Eel. Humid | Total flies 



Percentage 

 kiUed 



CaCl^ added to dry the air 



Air not treated 



Wet blotting paper as base 



Wet blotting paper as base and top 



791° 

 74-9° 

 77-0° 

 76-3° 



72% 



76% 



81% 

 93% 



229 



125 



66 



90 



94 

 89 

 47 

 17 



(iii). Experiments with baited formaldehyde under saturated conditions were 

 made. It was found that the baits about doubled its efficacy, but the results were 

 stiU very unsatisfactory. The details are given below in Table IX. Sugar in water 

 was the bait and the formaldehyde was employed in various strengths, being supphed 



