LEPIDOPTERA 



573 



parts of each wing that are subjected to the greater strain during flight; and is 

 more perfect in swift-flying species than in those of slow flight. 



The taxonomic value of these differences in the arrangement of the scales of 

 the wings of the Lepidoptera, and also of the different types of scales found in 

 different divisions of the order, was investigated by Professor Kellogg ('94), to 

 whose extended account the reader is referred for a discussion of this phase of the 

 subject. 



A secondary use of the scales of the Lepidoptera is that of ornamentation; 

 for the beautiful colors and markings of these insects are due entirely to the 

 scales, and are destroyed when the scales are removed. 



The various colors of insects and of other animals are produced in quite 

 different ways ; and classifications of these colors have been proposed based on the 

 methods of their production. The literature of this subject is too extensive to be 

 referred to in detail here. A most enjoyable popular account is given by Pro- 

 fessor Kellogg in his "American Insects" (Kellogg '08, pp. 583-614) and a de- 

 tailed analysis of the methods of the production of color is given by Professor 

 Tower in his "Colors and Color- Patterns of Coleoptera" (Tower '03). 



Following the classification of Tower, the colors of the scales of the Lepi- 

 doptera may be either chemical, physical, or chemico-physical. The chemical 

 colors are produced by pigments in the scales; the physical colors are produced 

 either by reflection, refraction, or diffraction of light; and the chemico-physical 

 colors are produced by either a reflecting, refracting, or diffracting structure 

 overlying a layer of pigment. There are also what Tower calls combination 

 colors due to a combination of the causes just mentioned. 



As the production of colors by pigments is the most obvious method in nature, 

 it is the one to which the colors of the Lepidoptera are commonly attributed. 

 But it is now well known that a large proportion of the most beautiful colors of 

 these insects are either physical or chemico-physical; this is true of the various 

 metallic and iridescent colors so commonly found in butterflies and many moths. 



Explanations of the methods of production of physical colors are given in text- 

 books on physics; it is, therefore, only necessary here to point out a feature in 

 the structure of the scales of Lepidoptera that results in the production of these 

 colors. This feature is the presence of the fine longitudinal striae described 

 above. When the striae are very fine and close together they act in thesame way as 

 does a diffraction grating, producing the beautiful iridescent colors. Kellogg 

 ('94) found that on certain scales from a species of Morpho the strias were from 

 .0007 mm. to .00072 mm. apart, or at the rate of about 35,000 to an inch. 



The fact that certain colors are due to the way in which light is reflected 

 from the scales can be shown by the following experiment. Place on the stage of 

 a microscope the wing of a bright blue butterfly, and shade the specimen so that it 

 is viewed only "by transmitted light from the mirror of the microscope; when 

 examined in this way the blue color will be absent. This is due to the fact that the 

 light passing directly through the scales is not broken up, and only the colors 

 produced by pigment are visible. 



There is still another function of the scales of Lepidoptera; they may serve as 

 the outlets of scent glands. As the scales that serve this purpose are found 

 chiefly on the wings of males, they have received the special name of androconia, 

 signifying male dust. See page 100. 



In the suborder Jugatas and in 

 some of the more generaHzed 

 families of the suborder Frenatffi, 

 there are, in addition to the more 

 obvious setse and scales, many 

 very small, hair-like structures, 

 which differ from setse in being di- 

 rectly continuous with the cu- 

 ticula, and not connected with it 

 by a joint (Fig. 710); these are 

 termed the fixed hairs or aculecB. 





Fig. 710. — Part of a wing of an aculeate 

 moth, with most of the scales re- 

 moved so as to expose the aculeae. 



