A pair of closely linked genes controlling high scutellar chaeta number in Drosophila.

1. A line selected for high scutellar chaeta number reached a mean of about 16 chaetae in females and 13.5 in males at the 69th generation of selection following an accelerated response to selection which commenced at generation 65 and added five chaetae. 2. The accelerated response can probably be explained in terms of two recessive high chaeta number genes 1.05 cM apart, and which are located between po and vg on chromosome II. The gene closest to vg was found to be scabrous, sca, which causes rough eyes when homozygous and has a pleiotropic effect on scutellar chaeta number. The gene was found in one of the strains used in setting up the selection lines. 3. The results are discussed in relation to other theories of control of the scutellar chaeta system.