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.
The size of the adult fly of D. ampelophila varies greatly according to the amount of nourishment obtained by the larva. After the fly emerges its size remains nearly constant, as in many insects. Two races have,
however, been separated by Bridges that are different in size as a result of a genetic factor. The first of these, called dwarf, is represented by figure 55, (b).
The race is minute, although of course its size is variable, depending on food and other conditions. The same figure shows the presence of another factor, "sooty", that makes the fly very dark. Maroon eye color might be here represented, due to still another factor.