572 



AN INTRODUCTION TO ENTOMOLOGY 



Fig. 708 -Scale 

 of S er y da 

 c onstans . 

 (After Kel- 

 logg.) 



It is well known that these scales are merely modified setae. 

 That is, they are setae which, instead of growing long and slender 

 as setae usually do, grow very wide as compared with their 

 length. Every gradation in form can be found, from that of 

 the ordinary seta, which occurs most abundantly upon the body, 

 to the short and broad scale, which is best seen upon the wings 

 (Fig. 707). This fact was pointed out by Reaumur nearly two 

 hundred years ago; and in recent times the morphological iden- 

 tity of setae and scales has been established by studies of their 

 development. Mayer ('96) gave a complete account of the de- 

 velopment of scales and illustrated his paper by most excellent 

 figures of all stages of this development. 



The structure of scales is what would be expected from the 

 fact that they are modified setae, the scales, like setae, being 

 hollow; and the manner of their attachment to the cuticula of 

 the body and its appendages is the same as that of the setae, each 

 scale being provided with a pedicel which fits into a cup-like socket 

 in the cuticula. 



A striking feature of the scales of Lepidoptera is the mark- 

 ings that exist on their exposed surface. These may consist merely 

 of many very fine longitudinal ridges (Fig. 707); or they may 

 be series of transverse ridges between the longitudinal ones (Fig. 

 708). 



the 

 the 



Fig. 709. — Cross-section of scales of Par- 

 nassius sminlheus. (After Kellogg.) 



A cross-section of certain scales indicates that 

 ridges are produced by foldings of the outer wall {i. e 

 wall of the scale that is exposed 

 when the scale is in place on the 

 wing). Figure 709 represents 

 cross-sections of a scale illustrating 

 this condition. In some scales, 

 however, the lumen of the scale 

 has been filled to a considerable 

 extent by chitin, and the origin of 

 the ridges is not so obvious. 



The scales of the Lepidoptera were probably developed from that type of 

 setae known as clothing hairs, and were primarily merely protective in function. 

 This is doubtless their chief, if not only, function on most parts of the body, 

 where they form a very perfect armor. 



The development of ridges on the surface of scales adds greatly to their 

 stiffness, and thus increases their efficiency as a protective covering, as the 

 corrugations in the sheets of iron used for covering the sides of buildings add to 

 the stiffness of the metal. 



Upon the wings a covering of rigid scales would serve not merely to protect 

 the wings but would tend to stiffen them, and thus arose a secondary function of 

 scales which has resulted in the perfecting of their arrangement upon the wings in 

 the more specialized members of the order as already indicated. 



There are great differences among the insects of this order regarding the regu- 

 larity of the arrangement of the scales upon the wings. With some of the more 

 generalized moths the scales are scattered irregularly over the svuface of the 

 wings. But if a wing of one of the more specialized butterflies be examined with a 

 microscope, the scales will be found arranged in regular overlapping rows; the 

 arrangement being as regular as that of the scales on a fish or of the shingles on a 

 roof. Figure 706 represents a sm.all portion of a wing of one of the more special- 

 ized butterflies, where the arrangement of the scales is most perfect. In the 

 upper part of the figure the membrane is represented with the scales removed. 



Even in those insects in which a very perfect arrangement of the scales upon 

 the wings has been attained, great differences in the degree of perfection of this 

 arrangement exist in the two wings of the same side and in the different parts of 

 the same wing. The arrangement is most perfect in those wings and in those 



