PARASITES  AND  ENEMIES  OF  DROSOPHILA.                   17

run through a "potato ricer," and then added to the agar solution.
Some workers add sodium hydroxide or carbonate to the mixture, but
it is best not to add enough to cause an alkaline reaction to litmus.
While still hot the solution is poured into the culture bottles and
allowed to harden. Half-pint milk-bottles are very convenient for
this purpose. About 50 c. c. of banana agar to a bottle will give good
results. A very little powdered yeast is sprinkled over the surface of
the agar in each bottle. A piece of absorbent paper (paper toweling
is usually used) a few inches square is added, and the bottle is stoppered
with cotton. The flies may be introduced as soon as the bottle cools;
the bottle should not be kept longer than 48 hours before using.

This culture medium has been found very favorable for many
species. Drosophila melanogaster, D. simulans, D. obscura, D. funebris,
D. immigrans, D. virilis, and D. willistoni have all been bred on it
very extensively in pairs. Mass-cultures of numerous other species,
including members of the genera Scaptomyza and Chymomyza, have
been found to thrive on it.

V. PARASITES AND ENEMIES OF DROSOPHILA.

Protozoan parasites: Chatton and his coworkers (1908, 1911, 1912)
have described flagellate parasites from the digestive tract and Mal-
pighian tubules of the European Drosophila confusa Staeger. These
are described as probably belonging to four distinct genera; but
Minchin (1912, Introduction to the Study of the Protozoa) interprets
them as different stages of the same parasite, which he places in the
genus Leptomonas. The effects produced on the hosts, and whether
or not other species of Drosophila have the same or similar parasites
are points not yet known.

Fungus parasites: Thaxter (1895, Mem. Amer. Acad. Arts and Sci., 12^
300; 1914, Botan. Gaz., 58, 235; 1917, Proc. Amer. Acad. Arts and Sci.,,
52, 699) has described several fungi that are external parasites on
species of Drosophilinse. These fungi belong to the genera Muiaria
(Hyphomycetes) and Stigmatomyces (Laboulbeniales). Each of these
genera is recorded both irorfuDrosophila and from Leucophenga. The
effects on the hosts are apparently unknown.

Nematodes and mites: Small mites are frequently to be found
crawling over living specimens of Drosophila. When numerous they
are apparently harmful, but I have not determined the manner in
which they affect the host.

Soil-living nematodes often occur in enormous numbers in fruit
exposed on the ground, and Drosophila fed on such fruit sometimes
has tangled masses of worms on its legs, wings, and bristles. These
appear as opaque white masses. They are injurious in that they
impede the movements of the fly and make it sluggish.