16 THE NORTH AMERICAN SPECIES OF DROSOPHILA. 



mushroom and concentrated banana were more efficient as food than 

 was unconcentrated banana, but much inferior to yeast (Baumberger). 

 Some yeast cells pass undigested through the body of the larva. These, 

 and others adhering to the surface of the larva, are scattered through 

 the fruit in which the larva burrows, and reproduce there. Thus a 

 loose symbiosis exists between Drosophila and yeasts (Baumberger). 



NATURAL FOOD OF DROSOPHILA LARV/E. 



The larvae of Drosophilinse may, in general, be termed scavengers. 

 As we have seen, they are probably primarily yeast-eaters, and the 

 material in which they occur is favorable for their development only 

 in so far as it forms a good culture-medium for yeasts. On this view 

 the marked specific differences in food-habits might be due to differ- 

 ences in the olfactory reactions of the adults, causing them to be 

 attracted to or to oviposit on different substances, or to a difference 

 in the kind of yeast (or bacteria) favored as food. Only a careful 

 study of some of the different species can show what is the correct 

 interpretation of these food habits. 



The genera Titanochceta, Acletoxenus, and Gitona, and the "inversa 

 group" of Drosophila, are apparently true parasites. The members of 

 the genus Scaptomyza are, primarilj^ but not invariably, leaf-miners. 

 Leucophenga larvae are most often to be found in fleshy fungi, and 

 Zygothrica and Mycodrosophila are, so far as known, limited to that 

 food. Chymomyza species usually breed in fruit or in the sap of 

 bleeding trees. Within the large and varied genus Drosophila occur 

 many and diverse food-habits. Each species has a characteristic type 

 of food, not always adhered to, but nevertheless usual. These may be 

 roughly classified as follows : 



1. Parasitic on cercopids: D. inversa, D. paradoxa. 



2. Corollae of large flowers: D. florcB, D. lutzii. 



3. Fungi: D. guttifera, D. putrida, D. transversa. 



4. Decaying fruit : Most of the common species. 



5. General scavengers, i. e., rotten potatoes, excrement, stale formalinized meat, etc.: 



D. busckii, D. funebris. 



The last two types are not as distinct as the others. Such forms 

 as D. caribbea, D. hydei, and D. repleta are intermediate between these 

 two types. They are commoner about fruit than the members of 

 type 5, but are attracted to excrement and other substances of the 

 kind about which type 5 species occur. All three of these have been 

 bred from fruit. 



BANANA AGAR. 



Laboratory cultures of Drosophila are usually reared on banana 

 agar. This is made up by taking equal amounts (by weight) of water 

 and banana, with enough agar to make 1 per cent of the total. The 

 water is boiled and the agar dissolved in it. The banana is usually 



