TAXONOMY. 89 



Evidently the action of natural selection has tended 

 towards uniting the two wings of each side during 

 flight in order to insure their synchronous action. 

 The frenulum and the frenulum hook. Compound 

 frenulum. Simple frenulum. Structure of simple 

 frenulum. Theory as to origin of frenulum. Sexual 

 differences. Results of experiments with trap lan- 

 terns. Superseding of frenulum by a large develop- 

 ment of the costal area of the hind wings. Last stages 

 of the frenulum. 



Lepidoptera which have neither frenulum nor a 

 large development of the costal area of the hind wings. 

 The jugum. Theory regarding the relation of the two 

 pairs of wings in the primitive lepidoptera. Division 

 of the order into jugatse and frenatsR. This division 

 coincides exactly with the primary division based on 

 the relative reduction of the radius and anal area of 

 the hind wings, as indicated above. 



3. The clothing of the wings. Variations in the 

 form of this clothing, hairs, and scales. Variations 

 in the arrangement of the hairs or scales. Function 

 of this clothing. Absence of cross veins in wings of 

 lepidoptera. Determination of the more generalized 

 form of clothing. Relative degree of specialization of 

 scales on upper and lower sides of wings, on fore and 

 hind wings, and on the proximal and distal portions 

 of the same wing. Minute structure of scales. Prim- 

 ary use of the ribs on the scales. Secondary use of 

 these ribs in the production of interference colors. 

 Examination of wing of blue butterfly with reflected 

 light and with transmitted light. 



