WING, WING-SCALES, AND THETB PIGMENTS IN BUTTERFLIES AND MOTHS. 199 



Pieridae are due to uric ackl. Mayer quotes this, and yet seems to 

 have failed entirely to apply the self-evident deductions to his own 

 observations on the white scales. Throughout his paper we read 

 nothing of the pigments in the white scales of certain butterflies ; 

 but the general assumption is more than once expressed, that the 

 colour of all white scales is due to their being devoid of pigment and to 

 the reflection of light. Hence the justice of Chapman's criticism 

 {Ent. life., ix., pp. 78-79) becomes still more evident. Hopkins not 

 only isolated this white pigment, but proved that under certain con- 

 ditions it could be readily changed into yellow. It happens that the 

 white Pierids, which have become mimics of the orange Nymphalids, 

 have really changed their colour from white to orange. The chemical 

 change, Hopkins has shown, is easy. Yet we know nothing as to how 

 it has been brought about. 



Hopkins has demonstrated that the pigment-factor in the wings 

 of the Pieridae was of an excretory nature, containing uric acid. The 

 production of the yellow Pierid pigment may be obtained by heating 

 uric acid with water, in sealed tubes, at high temperatures. This 

 " lepidopteric acid," as the yellow Pierid pigmentary matter is called, is 

 closely related to a red product, easily obtainable. To the practical 

 entomologist it will at once occur that white, yellow and red are the 

 three colours that function chiefly in the ornament of the I'ieridae, 

 and its importance is at once evident. The uric acid derivatives, 

 however, appear to be confined to the Pieridae among butterflies, for 

 when a Pierid mimics an insect belonging to the Xi/)iiphalidae, the 

 pigments are chemically quite distinct. Griffiths {( 'omptes Pie)idiis 

 Acad. Sci. Paris, cxv., pp. 958-959) shows that the green pigment 

 found in certain Papilionid, Hesperid and Nymphalid butterflies, 

 as well as in certain Noctuid, Cleometrid and Sphingid moths, also 

 consists of a derivative of uric acid, which he also calls "lepidopteric 

 acid" (Cj^-^ Hj^Q Az^ Ny Oj^^). By prolonged boiling in HCl it is 

 converted into uric acid. 



Since it is the modification of these pigments, into whose origin 

 we have been enquiring, that produces variation in the colours of 

 insects, I would return to the text on which I have repeatedly preached 

 during the last few years, viz., that all changes of colour in the scales 

 of insects are but outward manifestations of the living activities and 

 physiological processes of the animal in its earlier stages — egg, larva, 

 pupa — or some interference with the normal course of these activities, 

 or with its usual conditions of development. I need not here repeat 

 what I have already published^'-' on these heads, but the truth of which 

 is being substantiated by recent ei:bquiry, and it is quite evident that 

 in studying variation we must consider the vital processes of the 

 animal, and the peculiarities under which they are carried on, if we 

 are to get a true conception of the actual causes of variation. 



Mayer farther states that, " in connection with the phenomena of 

 pigmentation, it is interesting to note that, while uric acid may easily 

 be demonstrated by the muroxide test in the fluids of the alimentary 

 tract of the pupae of the Satuntiidae, it is never present in the hiemo- 



*Brit. Noctuae, vol. ii.,pp. i — xviii. " Variation considered biologically " (Ent. 

 Rec, vi., pp. 181, et seq.). " Pupal development and the colour of the resulting 

 imago" {Ent. liec, iv., pp., 311 — 315), etc. 



