352 THE AMERICAN NATURALIST [Vol. XLVIII 



chals among these is 79.5. Among the 1,206 flies of strain 

 1913— A there are but three post-dorso-centrals gone. 

 This latter is a highly reduced strain as regards post- 

 aerostichals, lacking 1,081. This great reduction in 

 acrostichals seems not appreciably to have affected the 

 dorso-centrals, a fact which argues for the independence 

 of the factors controlling the distribution of these two 

 sets of bristles. 

 Thus for flies recorded in Table I we have 

 Post-acrostichals lacking _ 79.5 



Number of Flies ~ ~ 2273 = ' 03 ' 

 One post-dorso-central lacking. 

 For flies in strain 1913— A (Table III) we have 

 Po^^C£o^ticiiaJsJa^^ _ 1081 



Number of Flies ~ 1206 = °' 89 ' 

 Three post-dorso-centrals lacking. 

 Among the 3,238 flies recorded in Tables I and III only 

 tour post-dorso-centrals are lacking, while among the 810 

 flies of strain 1912-c (Table II) there are 13.5 lacking. 

 The lack of post-acrostichals in this latter strain is 37. 

 1 here are 9.5 dorso-centrals lacking in the progeny of the 

 trio, cF a among which there are only seven post-acros- 

 tichals lacking. 



Thus we see that lack of post-dorso-centrals is in no 

 way correlated with lack of post-acrostichals, but is evi- 

 dently governed by distinct factors. 



Variation by Addition of Bristles 

 A strain of Lucilia sericata, 1913— E showed some 

 interesting variations chiefly iu the direction of addition 

 of bristles. The mother was normal (3, 3, 3, 3), taken at 

 the Bussey Institution, March 19, 1913. The detailed ac 

 count of the strain follows : tailed ™~ 



