438 Phylum Arthropoda 



MEDIA 



As shown by Guyenot (191 7), Baumberger (1919), and others, 

 Drosophila larvae feed principally on yeast. The technique of rearing 

 them consists, then, essentially in producing satisfactory media for 

 rearing yeasts that are also convenient in other respects. The usual 

 breeding place for wild specimens is decaying fruit (in the case of 

 D. melanogaster, the most used species), and this forms the most easily 

 used laboratory medium. Various kinds of fruit may be used, but 

 banana is the most satisfactory. Fresh, ripe bananas are peeled, placed 

 in a yeast suspension, and allowed to stand for about a day, then 

 drained and about 25 grams placed in a bottle; pint or half-pint milk 

 bottles are convenient. A folded square of filter paper (paper toweling 

 is less expensive and equally good) is added to soak up surplus moisture, 

 and the bottle stoppered firmly but not too tightly with cotton. Adult 

 flies are introduced, and should produce a new generation without fur- 

 ther attention in 8 days or more, the time depending upon temperature. 



The yeast suspension used may be started with ordinary commercial 

 yeast cakes, but it is better not to make a new suspension each day. 

 Routine procedure is as follows, where bottles are being made up fre- 

 quently. Each day the banana is removed from the jar, and new 

 banana for use the next day is added. The container (unwashed) is 

 filled with water and shaken, so as to wet all the surface of the banana. 

 Nearly all the water is then drained off, and the jar set aside. Banana 

 so treated is at its best after about 24 hours, and rapidly becomes less 

 satisfactory after two days. 



One of the early modifications of this technique was the stiffening of 

 the medium by the use of agar. This method was described in detail 

 by Bridges (1921); it is still preferred by many workers. Fresh ripe 

 bananas are peeled and pressed through a "potato-ricer" or coarse sieve. 

 For each 100 grams of peeled banana use 100 cc. of water and 2 grams 

 of agar-agar. Add agar to the water and heat until the agar is dissolved 

 (the solution may be hastened by stirring in fresh water after boiling 

 begins). Remove from flame, and at once stir in the banana pulp. 

 Pour about 50 cc. of hot medium into each half pint bottle. Add com- 

 mercial yeast (either sprinkle a small pinch of dry yeast, or add a drop 

 of a water suspension of fresh yeast cake). Add paper toweling and 

 stopper with cotton. Such bottles are ready to use as soon as they are 

 cool; they should not be used after two days. Before flies are added it 

 is desirable to remove a plug of material entirely to the bottom at one 

 edge; otherwise fermentation may cause gas to accumulate under the 

 food and force it out of the neck of the bottle. A corner of the paper 

 may be pushed into this hole to prevent the formation of a well of liquid 

 in which adults might drown. 



