In this paper a new mutant character of the larva of the silkworm moth is described. This character manifests itself especially by the appearance of a superfluous pair of abdominal legs on the second adbominal segment (Fig. I, B). The superfluous legs may appear also on the first abdominal segment, though this is very rare. The mutation, therefore, has in all six or even seven, though rarely, pairs of abdominal legs. The abnormal legs, though of normal size in the earliest stage, later, in the full grown larva, are considerably smaller than the normal legs, as they do not develop as fully as the normal legs. The mutation also has a slightly larger number of crochets in the abdominal legs.
In regard to inheritance this mutant character behaves as an ordinary mendelian dominant. The homozygous form, however, has one feature very different from the heterozygous form. The heterozygous form is characterized by the superfluous legs alone, while the homozygous form shows, in addition to the abnormal legs, an exceptional pair of the semilunar markings on the third abdominal segment (Fig. I, C). Accordingly, the homozygous form has two pairs of the semi-lunar markings, while the normal animal has only one.
If the animals with abnormal legs are inbred, there appear in the next generation, approximately in the proportion of 1:2:1, three types of individuals: those with normal legs and markings, those with abnormal legs and those with both a superfluous pair of legs and semilunar markings. Attempts to obtain a form with two pairs of the semilunar markings but without the superfluous legs have so far been unsuccessful. The mutation is accompanied by a slight lethal action even in the heterozygous form. As to the lethal effect, the homozygote is not thought to differ greatly from the heterozygote.
