actor, called antler,
insufficiently studied as yet.

In the third figure (c) the wings turn up at the end. This is brought about
by the presence of the factor called jaunty.

In the fourth figure the wings are long and narrow and several of the veins
are unrepresented. This character, "strap", is very variable and has not
yet been thoroughly studied. On the thorax there is a deep black mark
called trefoil. Even in the wild fly there is a three pronged mark on the
thorax present in many individuals. Trefoil is a further development and
modification of this mark and is due to a special factor.

In the fifth figure (e) the wings are arched. The factor is called arc. The
dark color of the body, and especially of the wings, indicates the factor
for black.

The sixth figure (f) shows the wings "curved" downwards. In addition there
is present a minute black speck at the base of each wing, due to another
factor called speck.

In the seventh figure (g) the wing is truncate. Its end is obliquely
squared instead of rounded; it may be longer than the body, or shorter when
other modifying factors are present. The mutation that produces this type
of wing is of not infrequent occurrence. It has been shown by Muller and
Altenburg that there are at least two factors that modify this
character--the chief factor is present in the second chromosome; alone it
produces the truncate wing in only a certain percentage of cases, but when
the modifiers are also present about ninety percent of the individuals may
show the truncate condition of the wing. But the presence of these factors
makes the stock very infertile, so that it is difficult to maintain.

In the eighth figure (h) the legs are shortened owing to the absence of a
segment of the tarsus. The stock is called dachs--a nickname given to it
because the short legs suggested the dachshund.

_Group III_

In figure 55, (a), a mutant type called bithorax is shown. The old
metathorax is replaced by another mesothorax thrust in between the normal
mesothorax and the abdomen. It carries a pair of wings that do not
completely unfold. On this new mesothorax the characteristic arrangement of
the bristles is shown. Thus at a single step a typical region of the body
has doubled. The character is recessive.

[Illustration: FIG. 55. Group III. (See text.)]

The size of the adult fly of D. ampelophila varies greatly according to the
amount of nourishment obtained by the larva. After the fly em