Drosophilidae 443 



light, with a frosted 100-Watt lamp, makes a satisfactory source of light. 

 A spherical flask of water should be placed in front of it, to serve as a 

 heat screen ; otherwise the flies will not remain etherized so long and may 

 even be killed by heat. The water in the flask may be colored by any 

 of the methods usual with microscopists. The flies are manipulated on 

 the plate by various instruments: a fine brush, blunt forceps, or a special 

 instrument known as a "fly-pusher." This is simply a pointed blade of 

 soft metal with at least one straight edge, usually curved to suit the 

 taste of the worker. This may be used for manipulating individual flies, 

 and also for raking whole groups of them into rows or off the plate. 



When the flies are discarded they are killed, since to release them into 

 the room increases the dangers of contaminating the cultures and also 

 favors the spread of mites. This is done by pouring them into a bottle 

 filled with waste alcohol, kerosene, or some heavy oil (Muller recom- 

 mends crank-case oil drained from an automobile). 



In making up new cultures it is, for many purposes, desirable to cross 

 known individuals. This involves the procuring of unmated females. 

 Mating may occur before the females are 24 hours old, especially if old 

 males are present. The usual procedure is to select pale large females, 

 which can with a little practice be easily identified as newly emerged 

 ones. There is a chance of error in this method, since such fe- 

 males occasionally will be found to contain sperm. Greater certainty 

 may be obtained by removing all the flies in the evening and then se- 

 lecting young females the next morning, since under these conditions no 

 old males will be present. The reason for suggesting these times of 

 day is that, under usual conditions, most of the emergence of the adults 

 from the puparia occurs in the early morning hours. Some workers pre- 

 fer to isolate pupae; but the labor involved here is disproportionate to 

 the added certainty of virginity. 



Oviposition may occur on the first day of adult life, but is commonly 

 deferred to the second; it continues at a high rate for about a week, 

 and then gradually decreases for a longer period, perhaps a month or 

 more. It follows that flies may be put into the culture bottles as soon 

 as they recover from ether; but my own practice is to keep them for 

 24 hours in vials. Ordinary shell vials are in any case commonly used 

 to keep them until they recover from ether, though they may be placed 

 in a paper cornucopia and put directly into the culture bottle. If they 

 are to be left in the vial for a day it is necessary to feed them. I use 

 a small piece of the "plug" taken from a culture bottle to prevent gas 

 accumulation, as described above. In transferring flies from vials to 

 culture bottles it is desirable to avoid rough handling, since this delays 

 oviposition somewhat. It is better to "pour" the flies toward the light, 

 rather than to shake them out. 



