Oil of Citronclla on two species of Dacus. 415 



vessels, each of the vessels containing a fragment of 



blotting-paper wetted with citronella oil. Four others 



similarly treated were confined in vessels without any 

 citronella. The result was as follows : 



With citronella. 2 amputated ^ put in 10.40 a.m., dead 

 at 11.0. 

 2 normal ^ put in at 10.20 a.m., dead at 

 11.0. 

 Without citronella. 2 amputated $ put in at 10.30 a.m., 

 June 16th. Both Hvely 7.30 a.m., 

 17th. Both found dead at 7.0 

 a.m., 18th, 

 2 normal ^ put in at 10.25 a.m., 16th. 

 One dead 7.30 a.m., 17th. Other 

 dead 7.0 a.m., 18th, 



This poisonous action may account for the fact that the 

 attractiveness of a rag is not proportional to the amount 

 of citronella with which it is wetted, a rag thoroughly 

 soaked being a less effective trap than one merely moist- 

 ened with a few drops of the oil. The flies prefer the 

 smell to be not too strong, but even when this is the case 

 it seems to have a stupefying effect on them, making them 

 dazed and lethargic, and quite impervious to ordinary 

 alarms. A very effective trap for them is a clean kerosene- 

 tin nearly filled with water to which ten or twenty drops 

 of citronella oil are added. The flies sit on the sides of 

 the tin, now and then approaching the water; as they sit 

 they get more and more stupid, and finish by falling into 

 the water and getting drowned. This way of catching 

 them is quite as effective as using citronella fly-papers, 

 and cheaper. On one occasion I exposed a glass tube of 

 half-inch bore and about three and a half feet long, in- 

 serted a piece of cotton-wool wet with citronella at one 

 end and corked it, leaving the other end open. Seven 

 zaaatus entered the narrow mouth of the tube and there 

 remained until they died, sitting in a line with their heads 

 toward the closed end of the tube. 



When in the neighbourhood of citronella the flies sit or 

 move here and there with wings expanded, often quickly 

 extending the proboscis, and now and then cleaning the 

 head with the fore- legs and rubbing them together. Not 

 infrequently they stand and rock their bodies to and fro, 



