176 T. H. MORGAN AND CLARA J. LYNCH. 



out at right angles at the sides of the body. The details of the 

 venation are shown in the three pairs of wings shown in Figs. B, 2, 

 3 and 4, while for comparison a normal (" long ") wing drawn 

 to scale is shown in Fig. I. 



The wingless wing represents in the main the basal portion of 

 the normal wing. The marginal vein is missing, but the cross 

 vein connecting the third and fourth longitudinal veins is often 

 present as an apparently marginal vein. The basal parts of all 

 five longitudinal veins are present. 



The balancers are much reduced in size and modified in char- 

 acter. A normal balancer is shown in Fig. 5 and one from a 

 wingless fly in Fig. 6. The terminal segment which is large in 

 the wild fly is represented by only a trace and the second segment 

 is reduced, but the basal segment approaches in size that of the 

 wild type. 



The wingless flies appeared in the ' truncate ' stock and 

 represented at first the wingless condition of the truncate type of 

 wing; owing to the low viability of the truncate flies the wingless- 

 ness was transferred to normal stock, but as it is difficult or 

 impossible to distinguish between Wingless of truncate stock and 

 wingless of long-winged stock the material was not at first quite 

 homogeneous except in so far as being wingless. There was also 

 present at first in the wingless stock another character, namely, 

 balloon wings, which appeared at about the same time as Wingless 

 in the truncate stock and proved as difficult to separate from the 

 Wingless as Truncate itself. As a result, balloon wings appear in 

 the first cross given below. The cross was made over a year 



ago by Miss E. M. Wallace. 



F 2 



Winged 9 , 1,136 



Pi F! 



Wingless cf Winged cf 

 Winged 9 Winged 9 



Winged cf , 1,038 

 Wingless 9 , 259 

 Wingless cT, 236 

 Balloon 9 , 12 

 Balloon <? , 3 



The sum of the winged and balloon is 2,189 an d that of the 

 wungless is 495, which gives a ratio of 4.42 to i. It is clear that 



