66 BRITISH LEPIDOPTERA. 



The pupae are exposed to the low and high temperature respectively, 

 at the period when the scale-pigments are undergoing differentiation 

 in the scales, from the haemolymph of the pupal blood. There is a 

 point at which this elaboration is carried on at a normally healthy 

 rate. At a temperature considerably above or below this normal point, 

 the pigment is developed abnormally, maybe never reaches its normal 

 condition (chemically), or, maybe, overshoots it. In either case, 

 abnormal conditions are produced, and, in these two instances, the 

 abnormality results in a melanic appearance of the insects. 



There are, of course, other forms of melanism which probably 

 have nothing in common with the cases already cited. One of these 

 is well represented by the ab. valesina of Dryas paphia, by the ab. 

 sufusa olArgynnis aglaia, etc., which are probably survivals of the old 

 form of the Argynnid female (vide, Entom. Rec, L, pp. 29-31). 



The production of albinism in Lepidoptera is not of very frequent 

 occurrence, still it occurs sufficiently often for the phenomenon to be 

 worthy of mention. It occurs in a more or less perfect manner in 

 species that rest on the ground, and which vary in tint according to 

 the colour of the soil upon which they rest. In Gnophos obscurata, 

 almost purely white specimens are often found in districts where the 

 insects rest upon the bare chalk, and the same is true of Eubolia bi- 

 punctaria, which has almost similar habits. These insects are, in their 

 typical forms, grey, i.e., their scales are — some black, others white. 

 The process of natural selection has weeded out the more conspicuous 

 (darker) examples in these localities, until a more or less white race 

 has been produced. It may be urged that these are not truly albinic 

 specimens, but they are exactly parallel in their mode of development 

 with some of the melanic forms to which we have previously referred. 



True albinic specimens, we take it, are such as those of Calli- 

 morpha hera, Triphaena pronuba, Catocala nupta, and other species 

 that have been recorded, in which the yellow or red pigment has failed, 

 and the scales have become white. In dealing with these specimens it is 

 evident we have a result based directly on physiological processes, for the 

 scales contain no pigment, the normal elaboration of the haemolymph ma- 

 terial having been largely or entirely suspended and the scales filled 

 with air. In our collection are specimens of Hemerophila abruptaria 

 and Hybernia aurantiaria exhibiting this phenomenon, and we believe 

 that the specimens of Sesia culiciformis in which the normal red (or 

 yellow) pigment of tbe abdominal belt is occasionally white, afford 

 a similar instance. 



Not very different is the cause which gives rise to the xanthic aberra- 

 tions, which are often included under the same head. In a paper, 

 "The genetic sequence of insect colours,"* we long since pointed out 

 that many instances of wbite coloration were due to an unstable pig- 

 ment in the cells, and that certain instances of black coloration were 

 also the result of highly differentiated pigment. These " whites " are 

 very rapidly changed to ochreous or buff under the influence of am- 

 monia, but regain their chemical equilibrium quickly on exposure to 

 the air. The embryonic scale is apparently filled with a secretion 

 from the haemolymph, which, in its first stage, becomes of a milky- 

 white coloration, afterwards changing rapidly to buff and ochreous- 



* British Noctuae and their Varieties, vol. ii., pp. i.-xvi. 



