FLUCTUATING VARIATIONS AND THEIR INHERITANCE 49 



of a single case in which Cu x is but ^ its normal length, ending freely 

 short of the outer margin (specimen Sub. 4, I left). 



The variation by loss in the anal series of the hind wing consists 

 of slight variations in length and character of the first anal vein which, 

 in the typical venation, is incomplete proximally. In 18 right and 20 

 left wings the distal half of the vein is perfect or normal. This distal 

 portion is in some wings either longer or shorter than ^2 the total 

 length of the vein from base of wing to outer margin ; in 4 right and 

 5 left wings, the vein is longer than the normal, while in 14 right and 

 10 left it is shorter than normal. In 9 right and 8 left wings there is 

 no true chitinization but some part of the vein's distal portion is repre- 

 sented by a thickening, fold or furrow. In 2 right and 2 left wings 

 the vein is ^ its normal length and is continued proximad to the wing's 

 base as a distinct fold. In one pair of wings the vein lies freely in the 

 cell Cu 2 , ending short of both proximal and distal margins of the 

 wing. 



The medial cross-vein of the hind wings varies by loss of parts as 

 does its homologue in the fore wings. 



The fact that variations by addition are of slight importance 

 (found in only 7 wings and in no case contributing any considerable 

 addition to the venation's total extent) as contrasted with the varia- 

 tions by subtraction or loss of venation in this functionally degenerate 

 organ is suggestive. It would seem to indicate that the variations in 

 this useless organ are characteristically of the nature of a breaking 

 down or degeneration of structures. It is interesting in this connection 

 to compare the conditions in these useless silkworm wings with those 

 found in the highly specialized and useful wings of the honey bees*, 

 in which addition of veins and cells was clearly characteristic of the 

 variation in their venation. 



Finally we reach the third group of variations in venation, namely, 

 variation by loss of the chitinization of the veins combined with the 

 substitution of persisting tracheae where the veins should be. This 

 variation occurs in the ist and 3rd anal veins and in the discal cell of 

 the fore wings and in M 2 , the medial cross vein, the 2nd anal vein, and 

 the discal cell of the hind wings. 



In the fore wing, there are tracheae in the discal cell in 5 right and 

 7 left wings, the ist anal vein is represented by a trachea only in I right 

 wing ; the 3rd anal vein is represented in part of its length by a trachea 



* See Kellogg and Bell, Studies of Variation in Insects, Proc. Wash. Acad. Vol. VI, p. ai*. 



