160 BIOLOGICAL LECTURES. 



the haemolymph consists of egg-albumen, globulin, fibrin, xan- 

 thophyll, orthophosphoric acid, iron, potassium, and sodium. 



When we consider the remarkable complexity of form and 

 diversity of color found in the scales of Lepidoptera, it appears 

 that some very potent factor must have operated to maintain 

 them so universally upon the wings of these insects. In the 

 first place, it is interesting to know whether the presence of 

 scales upon the wing serves to aid the insect in its flight. In 

 the act of flying it is obvious that the wing has two chief func- 

 tions to perform, i.e., it must beat downward against the air, 

 and in the performance of this action a relatively high coeffi- 

 cient of friction between the air and the wing may be of advan- 

 tage. But in addition to this, the wing must glide through the 

 air, and in doing this a small coefficient of friction is desirable. 

 There must, therefore, be an optimum coefficient of friction 

 which lies somewhere between these two. I tested the coeffi- 

 cient of friction between the wing and the air in the case of 

 several species of moths and butterflies, the wings being 

 mounted upon a short, light pendulum, and the damping of its 

 oscillations carefully observed. By this method it was found 

 that the coefficient of friction of the wings remained unaltered, 

 even when all the scales were removed from the wing mem- 

 branes. We may therefore conclude that the presence of the 

 scales does not aid the insects in flight, and it therefore seems 

 probable that the wings of the scaleless ancestors of the Lepi- 

 doptera had already attained to the optimum coefficient of 

 friction before the scales appeared, and that this coefficient 

 remained unaltered during the time when the scales spread 

 over the wing membranes. 



As the scales do not aid the insects in flight, it appears that 

 they must have been maintained and developed because their 

 presence granted some other advantage to the insects, probably 

 mainly because they displayed colors which may have served to 

 aid the insects in their struggle for existence. 



If one examine the colors of the wings of Lepidoptera by 

 means of Maxwell's color disks, it is found that nearly all 

 of the colors are quite impure ; that is to say, they contain a 

 considerable percentage of black. For example, the white 



