Drosophilidae 439 



This medium is much more convenient to prepare in large quantities 

 than is the fermented banana described above; cultures containing it are 

 more convenient to handle owing to the solid nature of the food; there 

 is less free moisture in which the adults may be drowned ; and its nature 

 is less dependent on the ripeness of the banana and therefore more 

 uniform. 



The banana agar is, however, rather expensive when hundreds of 

 cultures per day are needed; and it does vary with the condition of the 

 bananas used. These facts have led to the development of the cornmeal- 

 molasses-agar that is now the standard medium in this laboratory and in 

 several others. This medium has been described, together with a history 

 of its development and a discussion of its properties and of several vari- 

 ations, by Bridges and Darby (1933). The usual formula is as follows: 



The cornmeal is first wet thoroughly with some of the water. Of the re- 

 maining water, about two thirds is heated with the agar in it. As this 

 water begins to boil, more water is added and stirred in to hasten solu- 

 tion. The cornmeal and molasses are then added, and the mixture 

 boiled for 5 to 10 minutes. The remaining water is then added, to in- 

 crease the ease of pouring the hot medium into the culture bottles. With 

 this medium also it is necessary to remove a "plug" to prevent gas ac- 

 cumulation under the block of food. 



In the interests of economy it is usual to employ a low grade of corn- 

 meal that is sold as poultry feed ("fround maize" in England). Bridges 

 recommends cane molasses free of SO^; I am not convinced that SOj 

 is a serious drawback, but it is at least as well to avoid it. Corn syrup 

 may be used, and I have had good results from unrefined sugar substi- 

 tuted for molasses. The exact proportions may be varied within rather 

 wide limits without materially affecting the results produced. Many 

 workers prefer to measure, rather than weigh, the cornmeal since this 

 may be done more rapidly. 



Winchester (1933) recommends the addition of an extra supply of 

 dead brewer's yeast. This method enables more larvae to develop in 

 a small space, and also appears to help in the production of large, well- 

 grown specimens. For most purposes its advantages over well prepared 

 food supplied in sufficient amounts {i.e., in cultures in which relatively 

 few eggs are to be deposited) do not seem great. Similar results 

 may be obtained by adding a fresh supply of yeast after the larvae have 

 begun to develop. 



