VOL. III.] Proceed? nos of Societies. 85 



this minute powder tell us of the history of butterflies and moths? 

 An examination of various species discloses the fact that there is 

 considerable diversity both in the structure and distribution of the 

 scales. Among some species the scales are in the form of slightly 

 flattened hairs, irregularly scattered over the surface of the wings. 

 Between this type and the most specialized form of scale every grada- 

 tion can be traced; and it is found, moreover, that in species in 

 which the structure of the wings, antennse and other parts discloses 

 a lowly organization the irregularly disposed hairy form of scale is 

 present. Furthermore, it is found that the specialization of the scale 

 varies upon different parts of the wing. 



In order to understand the use of these scales it is necessary to 

 know something of the structure of the wing. The wing of the 

 dragon-fly is cut up by a net work of intersecting veins, but in 

 butterflies and moths the veins are fewer in number and cross-vein- 

 ing is rare. In the dragon-fly the mesh work of veins strengthens 

 the wing, while in the butterflies and moths the scales perform this 

 function. The more flat and regularly disposed the scales are, the 

 greater will be their strength. Accordingly any variation in the 

 direction of a flattened scale will be preserved by natural selection. 

 It is to be expected, moreover, that the greatest change will occur 

 in the region of greatest strain. It is found that this is indeed the 

 case, lor the scales are more flattened on the front than on the hind 

 wing, and at the tip more than at the base. As an additional strength- 

 ener, ridges have been developed along the scale. Incidentally, 

 these ridges have also been productive of a great variety of iri- 

 descent colors, by the interference of light. As soon as these color 

 effects began to manifest themselves, sexual selection would be in- 

 troduced as a factor in the modification of scales. 



Having followed out one line of development it is necessary to 

 correlate this with the evolution of other parts. The classification 

 of insects is based largely upon the structure of the wings. In lower 

 forms the wings are broad and far apart, while higher forms are dis- 

 tinguished by having them closer together and more compact. In 

 order to give still greater strength to the stroke of the wings a 

 bristle or clump of bristles known as a frenulum, is developed near 

 the base of the upper edge of the secondary wing. When consisting 

 of a bundle of bristles each one is a hollow tube, but when formed 

 of but one bristle it is composed of a number of tubes joined to- 



