APPENDIX 405 



should be gradual or there is danger of the bottles breaking. The 

 cotton wool stoppers may be sterilized at the same time. 



The food is poured into the bottles while still hot and fluid, preferably 

 through a fairly wide funnel of thin metal which gets hot and does not 

 cool the food and cause it to solidify. The layer of food should be 

 about J in. thick. Stopper the bottles and allow to cool, or place a 

 (sterile) duster over them. When food has solidified, put on the surface 

 of the food one or two drops of a thin suspension of live brewer's 

 yeast. At this stage two further refinements can be added (i). A strip 

 of absorbent paper (e.g. toilet paper) is pressed into the surface of the 

 food so as to form a cone with the open end upwards; when putting 

 etherized flies into a bottle they are placed on the paper, which prevents 

 them getting sttick to the surface of the food, and the paper is also 

 used by the larvae for pupation (2). A hole may be punctured in the 

 food to assist the escape of fermentation gases. 



d. Control of Pests 



The only pests are molds and mites. It is best to prevent these by 

 addition of preservative to the food. The best are Nipagin-M or 

 Moldex-A, about 0-15 per cent, which should be boiled with the food 

 for about three minutes. If cultures get mouldy, the flies may be 

 cleaned by transferring every day to fresh food. 



e. Temperature 



The optimum is about 25° C, but the flies do quite well at ordinary 

 room temperature. At 25° C. the life cycle is about a fortnight. 



Stocks kept at room temperature should be transferred to new food 

 about every six weeks, or rather oftener. It is best to keep the old 

 bottle in case the new one does not take. Stocks can be transferred by 

 simply shaking the flies from one bottle into the next. It is often con- 

 venient to remember that the flies are strongly positively phototropic, 

 and do not escape quickly from an open bottle if the bottom is held 

 towards the light. 



TECHNIQUE OF MAKING CROSSES 

 AND SCORING PROGENY 



a. Etherizing 



For sorting flies or selecting parents for crosses, it is necessary to 

 anaesthetize with ether. This is done most simply in an etherizer 



