5O INHERITANCE IN SILKWORMS, I 



in 20 right and 22 left wings, while in 13 right and 17 left wings the 

 3rd anal vein is represented by a trachea only. 



In the hind wing the medial vein is represented wholly or in part 

 by tracheae in 4 right wings ; in I right and I left wing the vein M 2 is 

 in part of its length a trachea only; in i right wing the 2nd anal vein 

 is a trachea only in its distal portion; there are tracheae in the discal 

 cell in I right and 4 left wings. 



In one case (the left hind wing of specimen Sub. 3, I) the tracheae 

 in the discal cell show an arrangement which might be interpreted as 

 throwing light on the ancient type of venation in the discal cell before it 



Fig. 2. Diagram showing relation of tracheal trunks to the radial and median 

 veins in the silk-worm moth, Bombyx mori. 



became a single cell. A single longitudinal trachea arises from the 

 base of the wing and forks at about the center of the cell into two 

 branches (M 1+2 and M 3 ). The cephalic branch forks again within the 

 discal cell, separating M x from M 2 , while the caudal branch meets and 

 fuses with the medial cross vein until M 3 again turns longitudinally 

 and continues as M 3 distally to the outer margin. (Fig. 2.) It is 

 only within the discal cell that the medial series is represented by 

 tracheae, the veins of the series being well chitinized outside the discal 

 cell. 



To sum up the variations in venation found in these functionally 

 degenerate wings of the silkworm, we find very little variation by 

 addition and no variations in the direction of specialization for a 

 strengthening of the wing skeleton. We find a very large amount of 

 variation by absence of certain veins or by loss of the parts of veins, 

 in some cases the loss being total, in some cases an imperfection in the 

 chitinization and in many cases the survival of tracheae as substitutes 

 for the missing veins. 



This loss of parts of the disused supporting skeleton of the wing 

 is, of course, exactly what we should expect to find in the light of 

 that degeneration of function which has become characteristic of silk- 



