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). 
v 
NATURAL FOOD OF DROSOPHILA LARVA. 
The larve of Drosophiline 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 Titanocheta, Acletorenus, and Gitona, and the ‘‘inversa 
group” of Drosophila, are apparently true parasites. ‘The members of 
the genus Scaptomyza are, primarily but not invariably, leaf-miners. 
Leucophenga larve 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: 
. Parasitic on cercopids: D. inversa, D. paradoxa. 
. Corolle of large flowers: D. flore, D. lutzii. 
. Fungi: D. guitifera, D. putrida, D. transversa. 
. Decaying fruit: Most of the common species. 
. General scavengers, 7. e., rotten potatoes, excrement, stale formalinized meat, etc.: 
D. busckti, 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. 
ar Whe 
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 
