INTRASPECIFIC VARIABILITY. 43 
The wing-vein indices, though known to be variable, nevertheless 
offer valuable aids in the identification of species. How variable they 
are may be gathered from the data in tables 5 and 6, which show the 
results obtained from measurements of D. melanogaster from a number 
of different sources, and of D. melanica from Massachusetts. 
TaBLE 5.—Fourth-vein index, D. melanogaster. 





No. of 
individuals. 
No. of 
Index. idietaales 

No. of | 
individuals. Index. 
Index. 



rw 
CO SIS Or 



Total No., 70. M = 2.353 + 0.017; o = 0.213 + 0.012; V = 9.08 + 0.58. 
TaBLE 6.—Indices of D. melanogaster and D. melanica. 






Melanogaster. Melanica. 
Costal pia f oD pen | Costal No. of Fourth- nee 
index. mor, index TBO yl ie aineen! indi- Svein era 
viduals. viduals. || viduals. index. viduals. 
| {| eee ences 
1.9 1 1.6 1 2.9 1 1.5 1 
2.0 2 Lat f 0 3.0 0 1.6 3 
2.1 2 1.8 1 Sul 0 Sef 5 
Qe 1 1.9 1 3.2 1 1.8 3 
Deo 1 2.0 1 Su3) 4 
2.4 1 2.1 1 3.4 1 
20) 2 2.2 1 Sia 0 
2.3 1 3.6 1 
2.4 0 3.7 a1 | 
| 2.5 | 1 | | 
TS ac a eae eas: Total... | Oveihy a sadioy. 2. eae iy 2 



The number of branches on the antennal arista is usually given as a 
specific character. It is also variable, as shown in table 7. In this 
table the terminal portion of the main axis is counted as a branch, 
so that an arista described as having five branches above and three 
below would be entered here under “‘9.”’ 
TaBLE 7.—Variation in number of aristal branches. 







Species. Source of stock. 6|7|8 | 9110/11| 12] Total. 
Dmelanocasterss ois ss he sae. Camp Jackson, 8. C...... ON ONE: Si 658 22/11/0)) SOs 80 
Berkeleya Cali cya: ONO LAZO Ss Ol On 40 
DIMMIPTANG Ao ols hts Arlington ys Mies) oo. ee OP OW MAN 28 36140 eZ 
LUNE DTIG! Mae) Aen alien AnnvArbor:, Mache: 5. 2'()02110)} (0), 9) 12) 117).10)), 40 
replete yeh ia eee ath ee INe@wiaWorka Neyer a) hee OLA Ga ON On ONO 18 
1D ead ate (2) aie ea NOS MTT ty Pe aca akelandblae. yo seen lad anlden weealp ON mONn OM Ole VLG 


Statistical constants for D. melanogaster, both races taken together: 
M = 9.042 + 0.038; o = 0.610 + 0.027; V = 6.74 + 0.29. 
