THE INTERNAL STRUCTURE OF THE LEPIDOPTEROUS PUPA. 81 



(Ent. Rec.,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 Pieridae, 

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

 ever, appear to be confined to the Pieridae among butterflies, for when 

 a Pierid mimics an insect belonging to the Nymphalidae, the pigments 

 are chemically quite distinct. Griffiths (Comptes Rendiis Acad. Sci. 

 Paris, cxv., pp. 958-959) shows that the green pigment found in certain 

 Papilionid, Hesperiid and Nymphalid butterflies, as well as in certain 

 Noctuid, Geometrid and Sphingid moths, also consists of a derivative 

 of uric acid, which he also calls "lepidopteric acid" (C 1X H 10 Az 3 

 N 8 O 10 ). By prolonged boiling in HC1 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, we would again reiterate what we have repeatedly asserted 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. We need not here repeat what we 

 have already published* on these points, but the truth of which is 

 being substantiated by recent enquiry, 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 further 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 to be present in the fluids of the 

 alimentary tract of the pupae of the Saturniidae, it is never present in 

 the haunolymph of the imago, nor can it be detected in the drab- 

 coloured pigment of the outer edges of the wings. The amount of 

 uric acid in the fluids of the alimentary tract of the pupa increases as 

 the pupa becomes older, so that the fluid which is voided upon emer- 

 gence is always strongly impregnated with it. In the case of Pieris 

 rapae there is no uric acid, either in the alimentary tract or haemolymph 

 of the larva, but it is present in the alimentary tract of the pupa. It 

 seems to me probable that the uric acid of the alimentary canal 



* Brit. Noctuae, vol. Li., .pp. i — xviii. " Variation considered biologically." — 

 Ent. Rcc, vi., pp. 181 ct seq. " Pupal development and the colour of the resulting 

 imago.''— Ent. Rcc, iv., pp. 1311-315, etc. 



