i66 



NATURE 



[August 3, 191 1 



sorv disc," or simply the "disc." From the apex ol the 

 lamina arise the cilia-like processes, which ma; convenienth 

 be termed the " fimbriae." This may serve as an example 

 of a form of plume-scale commonly found in the white 

 butterflies or Pierinae, but the scale in all its parts is liable 

 to considerable modification ; and, indeed, ii may be said 

 with truth that in no two species are the scent-scales 

 exactly alike. The diagrams will show how very diverse 

 are the form-; assumed by the plumules of these white 

 butterflies ; but in all of them may be observed under 

 various shapes tin lamina, fimbria;, footstalk, and acccssorv 

 disc. Th. scale is formed of chitin, a substance which 

 constitutes the outer covering, or external skeleton, as it 

 is called, of many insects, and which when met with in 

 bulk is of a hard and horny consistence, as may be seen, 

 for instance, in the forewings or " elytra " of beetles. 

 Chitin is practically a dead material, and there is no trace 

 to be found in the scale of any protoplasmic or living 

 matter. The granules which are present are probably 

 pigment granules, the presence of which in the ordinary 

 scales imparts to the wings their characteristic colour. So 

 far we have discovered no apparatus to which we may 

 attribute the production of perfume. 



But now let us examine the means by which the scale is 

 attached to the membrane of the wing. The point, or 

 rather surface, of attachment is the accessorv disc, which 

 fits into a cup-shaped depression in the wing-membrane, 

 which cavity, however, is generally not large enough to 

 admit the whole of the disc. In many species can be seen 

 an orifice in that part of the disc which is enclosed, when 

 the scale ii in rite, within the cup-shaped cavity or socket 

 just mentioned. And when the fimbria; are examined with 

 a very high power,, an appearance is seen in many species 

 which suggests that their free extremities are not closed, 

 hut open : that they are, in fact, minute tubes which put 

 the interior of the scale into communication with the outer 

 air. Now can we discover any means by which, say, 

 a vapour entering the disc by the orifice' in its buried 

 portion can lie conveyed through the scale and find 

 its way out through the patent extremities of the fimbria-? 

 It certainly appears that we can. Within the disc then is 

 generally visible a chitinous structure which often bears the 

 appearance of a convoluted tube ; the footstalk which forms 

 a bridge between disi and lamina is apparently not solid, 

 hut pervious. The lamina itself consists of two deli, a',. 

 chitinous layers, one of which may he called dorsal and the 

 other ventral, enclosing a flattened cavitv which contains 

 a certain amount of interstitial material. This latter take- 

 various forms in different species, but verv often presents 

 the appearance of a longitudinal striation, which in all 

 probability betokens the existence of fine parallel channels 

 or passages traversing the interior of the lamina side bv 

 side from base to apex. This longitudinal striation is 

 frequently obscured by the accumulations of granular 

 pigment: but in many cases there is a comparatively clear 

 area near the apex where the stria? can he fairly well made 

 out. and where they ran be seen to correspond in number 

 and position with the individual fimbria-. There is, then. 

 much reason to -oppose that the cavity of the lamina is 

 more or less completely divided into channels which- com- 

 municate in one direction with the fimbria;, and so, through 

 the orifices of the latter, with the outer air ; and in the 

 other direction through the fool, talk with the disc, and so 

 through the aperture of the disc with the socket ol the 

 wing-membrane and its underlying structures. We have, 

 therefore, some warrant for considering the seal.- to he a 

 piece nf apparatus not indeed for the manufacture, hut for 

 the distribution of sent: and to get some insight int.. the 

 mode of production of the latter, it is evident thai we 

 must pur-ue our researches into the structure of the winu 



itseff. 



It was noticed by Weismann more than thirty years ago, 

 and more recently by Gunther, that in the hypodermis, as 

 it is called, or cellular layer immediately underlying the 

 homogeneous surface-membrane of the wing, there occur 

 certain cells which appear to !»■ soecialised for the pro 

 duction of a secretion. Thesi - - II- were described and 

 figured by Gunther under the name of " Driisenzelle, " or 

 " gland-cells." In this diagram, which is copied from one 

 of Giinther's figures, we see two of these " gland-cells " 

 in direct connection with the sockets in the wing-membrane 

 into which fit the footstalks of two scale- parth Seen in 

 NO. 2\~i), VOL. 87] 



section. These -tales are not plume-scales, for they possess 

 no disc; but if secreting cells are found, as here, in con- 

 nection with scales of the ordinary kind, there seems to 

 be no reason why we should not also find them in relation 

 with plume-scales, supplying in that position the living and 

 working protoplasmic element by means of which the scent- 

 bearing secretion is elaborated. The clear spaces in these 

 cells of Giinther's figure are rather suggestive of the oil 

 or fat vacuoles seen especially in growing cells of adipose 

 tissue ; and it may be conjectured that the scent-bearing 

 secretion is of the nature of a volatile oil. In the case of 

 the ordinary stales the secretion may still be oily. Prob- 

 ably most of us know how difficult it is to wet a butterfly's 

 wing with water. This is no doubt partly due to the 

 mechanical conditions involved in the coating of minute 

 scales, but it is possible that, as in the case of the plumage 

 of aquatic birds, some additional power of resistance to 

 wet is afforded by the presence of an oily secretion, which 

 may be conveyed to the surface by the scales of ordinary 

 character. It is also possible, as has been suggested by 

 Weismann, that the secretion formed in connection with 

 the ordinary scales may bear an odour, though of a different 

 nature from that of the plume-scales, and, at least in 

 many cases, imperceptible by our senses. All this is a 

 matter of more or less probable conjecture, and it is very 

 clear that there is a good deal more work to be done 

 before we ran be sure that we know all about the various 

 functions of the scales and their associated structures. 



Before we go on to the next part of our subject I 

 should like to call your attention to some figures that will 

 be thrown upon the screen of various forms of plume- 

 scales. In these figures the chitinous sculpturing of the 

 scale will be seen. It differs in character from species to 

 species, but in all there is more or less visible a longitu- 

 dinal striation of the lamina, which we have seen reason 

 for interpreting as an indication of channels along which 

 pass the odoi iferous secretions or exhalations from the 

 gland-cells buried in the substance of the wing to the 

 fimbrise and so into the open. 



The question will no doubt have occurred : are these 

 plume-scales the only structures by which the scents of 

 butterflies are distributed? They are by no means the 

 only ones. There are many other methods of distribution 

 of these flower-like odours, some of which we can find 

 without going beyond the group of so-called white butter- 

 flies, or Pierincs. Visitors to the south coast of England 

 in the late summer and autumn months ran hardly have 

 failed to noiie. a very active butterfly of a tin.- bright 

 orange colour with a dark border, which is especially given 

 to haunting fields of lucerne and clover. This is the 

 butterfly commonly called the " clouded yellow," one of 

 the most conspicuous of the whites, or, as we ought rather 

 to say, the Pierine butterflies. In this insect we should 

 search for plume-scales in vain ; but on examining in a 

 male specimen the front edge of the hindwing where it is 

 overlapped by the forewing, we find on tin- upper surface 

 a patch of scales distinguished from their surroundings by 

 their lighter colour. The microscope shows that these 

 scales are of a different shape from those of the rest of the 

 wing, and are packed much more closely together; more- 

 over, instead of Icing nearly flat upon tin- wing, like the 

 tiles on a roof, they are set up on end. sometimes almost 

 at a right angle. When the wing membrane is denuded 

 of scales and examined with a high power, the situation 

 of the patch is easily recognisable by the crowding to- 

 gether of the sork.-ts for the insertion of the footstalks, 

 and also by the fact that trachea-, or air-tubes, are seen 

 to he leaving one of the main " veins " of the wing and 

 supplying this particular area, breaking up into smaller 

 branches as they go. 



In ordinary circumstances tin- scent of the clouded 

 y.-llow is not easily detected ; but if in a living specimen 

 the scales he scraped off one of the patches that have just 

 been described, they will in many cases he found to have 

 an odour which is somewhat like that of the garden helio- 

 trope, or " cherry-pie. " The South African clouded yellow, 

 which is much lik. ours, though quite distinct, has a 

 similar patch and a similar odour. The scent-producing 

 apparatus in these clouded yellow butterflies presents many 

 features of interest ; in the first place, the scent-scales are 

 crowded together into on.- small area, instead of being 

 generally distributed over the wing-surface as in the com- 



