Drosophilidac 441 



prefer not to make use of any paper at all, since it makes it very much 

 more difficult to be certain that all adults have been removed from a 

 bottle. 



The cotton plugs are more convenient to handle and have a longer 

 life if they are wrapped in cheesecloth. They may be used repeatedly, 

 if sterilized. A dry air sterilizer is useful for this purpose, or fumes of 

 formalin or of carbon tetrachloride may be used if plugs are thoroughly 

 aired before being used again. In cases of serious infection with molds 

 or mites (see below) it is perhaps safest to destroy all old plugs. Some 

 workers use the cardboard caps that dairymen employ, instead of cotton 

 plugs. These must have pin-holes pricked in them to allow air ex- 

 change ; they have been discarded in this laboratory because the pinholes 

 make the spread of mite infections too easy. 



Culture bottles may be kept at a wide range of temperatures, but be- 

 tween 1 8° and 2 6° C. is best for most purposes. Above this range breed- 

 ing is possible, with most strains, up to about 31° (Plough and Strauss, 

 1923); but above 27 is definitely suboptimal. The results of Young 

 and Plough (1926) show that the most marked effect of unfavorably 

 high temperatures is that of sterilizing the males. At low temperatures 

 oviposition is greatly reduced (practically ceasing below 15 ) and the 

 development rate is slowed down markedly, though it is possible for 

 the whole development to be completed at temperatures at least as 

 low as 12 . For many purposes it is necessary to control the tempera- 

 ture, since the development rate and many of the structural characters 

 of the adult vary with temperature. Bridges (1932) has described an 

 incubator adapted to Drosophila work. The design of such an incu- 

 bator is outside the scope of the present account, but two points may be 

 mentioned. Firstly, it is desirable to arrange, so far as possible, that 

 when the thermostatic control goes wrong the heating unit will go off, 

 not on, since too high a temperature is more harmful than too low. Sec- 

 ond, it must not be assumed that the temperature in the bottles is the 

 same as that in the incubator, since the fermentation of the medium 

 makes some heat. This source of error has not usually been taken into 

 account in the published results on temperature experiments with 

 Drosophila. 



In many regions summer temperatures are likely to reach the danger- 

 point. If the heat is not prolonged cultures may survive, but it is often 

 necessary to place the bottles in a cold-room or otherwise protect them. 

 Setting their bases in running tap water is often adequate. For routine 

 genetic experiments incubators set at 25 are usually employed; in this 

 laboratory stock cultures are kept at 19 , since the resulting decrease in 

 development rate saves labor by making it unnecessary to make up new 



