SCALES. 



[ 678 ] 



SCALES. 



exhibits a number of irregularly radiating 

 rugae or folds of the upper membrane of the 

 wing (PI. 34. fig. 26). The cups are arranged 

 in more or less regular transverse rows. 

 Each scale is composed of two superficial 

 laminae, enclosing a central lamina of struc- 

 tureless membrane, the surface of which is 

 highly polished. 



The scales are variable in form, both in 

 difTerent insects and in different parts of 

 the same insect, being oval, oblong, cordate, 

 obcordate, or cuneate, &c. (Pis. 1 & 34) ; 

 sometimes they are filiform or capillary 

 (PI. 34. fig. 27). Their free end is rounded, 

 truncate, toothed, or terminated by a num- 

 ber of hair-like processes ; and they are ar- 

 ranged like the tiles of a roof, overlapping 

 each other (fig. 26). 



The interesting markings seen upon the 

 scales vary considerably in different insects. 



The most common, as seen by transmitted 

 light, are longitudinal, simple, continuous, 

 parallel or slightly radiating dark striae or 

 lines (PL 1. figa. 6, 7, 8, 9 a) . These are met 

 with upon the scales of nearly all butter- 

 flies and many other insects. In some in- 

 sects the striae are not simple and conti- 

 nuous, but are made up of rows of smaller 

 striae in twos or threes meeting at an angle 

 (PL 34. figs. 28 b, 30 & 31)'. In others they 

 are composed of a number of bead-like dots, 

 or are interrupted, still preserving their ge- 

 neral longitudinal direction (PL 34. fig. 24) ; 

 or they are slightly undulate or irregular, 

 and give off short lateral branches (figs. 23 a 

 & 29). In others, again, they present dila- 

 tations in certain parts of their course (figs. 

 20 & 21). 



These longitudinal striae consist of eleva- 

 tions or ridges upon the surface, probably 

 representing folds of the upper layer or 

 membrane of the scale. They often project 

 slightly from the free end of 'the scale (PL 

 34. figs. 3 & 22); and when moistened, 

 bubbles of air may not unfrequently be 

 found imprisoned between the surface of 

 the scale and the cover, which, being con- 

 fined between two of the ridges, assume an 

 oblong form. They sometimes contain air, 

 which may be displaced by liquid (PL 34. 

 fig. 21). We have never been able to de- 

 tect tracheae in these folds or in the scales. 

 A minute conical point or spine sometimes 

 occurs in each of the dilatations when pre- 

 sent (fig. 20 a). 



In the scales of Podura (PL 1. fig. 12), 

 the striae consist of longitudinal rows of 

 minute wedge-shaped bodies. 



In addition to the longitudinal striae, on 

 most scales, especially when examined by 

 unilateral oblique light, are seen a number of 

 minute transverse striae (PL 1. figs. 7 & 9 ). 

 These are neither indications of ridges nor 

 depressions, but arise from the existence of 

 a number of pigment-granules situated be- 

 tween the two layers of the scale ; and the 

 appearance of striae has the same origin as 

 that in the case of the valves of the Diato- 

 maceae. This point is best examined in 

 brown or other dark-coloured scales. If 

 perfectly direct light be transmitted through 

 one of these scales, the transverse striae 

 vanish, their place being occupied by the 

 distinct and isolated granules of pigment 

 (PL 1. fig. 9 6) ; the scale should also be 

 immersed in balsam or liquid, to diminish 

 the effects of the refraction arising from the 

 inequalities of the surface of the scale. On 

 then transmitting unilateral oblique light 

 through the scale, the appearance of trans- 

 verse striae may be easily produced. 



The colours of the scales of insects arise 

 partly from iridescence, partly from the pre- 

 sence of pigment ; in general, the brilliant 

 colours depending upon the former, and the 

 more sombre hues upon the latter. The 

 darkness of the longitudinal striae is caused 

 by refraction ; for scales containing no pig- 

 ment appear perfectly white by reflected 

 light, although the striae may be very dark. 



Upon certain scales, other irregular, more 

 or less transverse curved striae exist (PL 34. 

 figs. 3 & 22) ; these appear to consist of 

 wrinklings or folds of the under membrane 

 of the scale. 



In examining the scales of insects, they 

 should be viewed both in the dry state, and 

 immersed in water, or oil of turpentine; 

 and both by transmitted and reflected light. 

 When the insects are pressed against the 

 slide to remove the scales, a number of 

 globules of oil adhere simultaneously to the 

 slide ; and when the cover is applied, the 

 scales often become partially or entirely 

 covered with the oily matter, producing an 

 appearance as if the upper layer of the scale 

 were removed, and rendering the markings 

 so pale and indistinct as to be apparently 

 absent. The appearance of transverse striae 

 is best produced by turning the mirror to 

 one side, so as to reflect unilateral light. 



A brief notice of some interesting insects 

 in respect to the structure of their scales is 

 given under the individual heads, as Cun- 

 cuuo, LEPISMA, MOBPHO, PODURA, Po- 



LYOMMATUS, PONTIA, TlNEA, &C. 



