Reactions of the Pomace Fly to Odorous Substances 52 1 



passage in the trough was 5 mm. deep, and 11 mm. broad and 

 extended from the near end, which was closed by a zinc shde C to 

 the square chamber at the far end. The trough was covered by a 

 glass plate D, which fitted to the lead closely enough to make it 

 practically air-tight. Against the near end of the trough was an 

 inverted cylindrical g^lass dish £, which served to hold the flies to 

 be tested. This dish was raised to the level of the glass plate by 

 a block of wood. Its chamber communicated with the passage of 

 the trough by a short inclined way F, which allowed the flies to 

 pass into the trough when the slide C was open. At the opposite 

 end of the trough and resting upon the glass plate was another 

 inverted glass vessel G, communicating with the trough by means 

 of a small hole H through the glass plate. This vessel served as 

 a reservior to hold the flies that had been tested. 



A fly creeping from chamber E along the trough to chamber G 

 passes between the open ends of the two traps, which were inserted 

 opposite each other through the walls of the trough at / and P. A 

 small glass tube piercing the far wall of the chamber at J was con- 

 nected by a rubber tube with a suction apparatus, by means of 

 which a current of air could be drawn through the trough at any 

 desired rate. The suction apparatus consisted of a large bottle 

 filled with water, closed at the top by a stopper with two holes. 

 Through one hole was inserted a bent glass tube, which served 

 as a siphon. The other hole was filled by a short glass tube which 

 connected with the rubber tube from J and served to admit the 

 air under external pressure to the partial vacuum formed by the 

 siphon. Reference to Figs. 2 and 3 will show that when the siphon 

 was allowed to run at a given rate, controlled by a clamp on the 

 rubber tube, a current of air flowed from E through the trough, 

 past the ends of the traps to the outlet J . The aim was to have 

 this air current carry all the escaping odorous particles away 

 from the mouth of the trap. To test the apparatus, hydro- 

 chloric acid was allowed to evaporate in chamber E and this gas 

 was drawn by the air current along the trough and past one of the 

 traps which was charged with ammonia water. White fumes of 

 ammonium chloride were formed at the mouth of the trap /, and 

 deposited along the path of the current. The dotted line in Fig. 2 



