74 BRITISH LEPIDOPTERA. 



beneficially, until it reaches 60°, and probably will not affect it preju- 

 dicially until it exceeds 66°. 



It is a fact that the largest, most vigorous, and most brightly coloured 

 specimens of Chrysophanw phlaeas are obtained in the temperate parts 

 of the Palaearctic area, and that, as we pass south, the insect becomes 

 less brilliant, darker, and often smaller. This tends to show that it 

 is one of those species that prefer an environment more like that of 

 our temperate climes, and that a higher temperature affects it more or 

 less prejudicially. The most easily-marked evidence of this prejudicial 

 action appears to be seen in the scaling, for, even in Britain, a very 

 hot summer, like that of 1893, always produces a fair proportion of 

 dark specimens, even in those localities where, in cooler seasons, the 

 colour is most brilliant. This is sufficient to prove that the range of 

 variation in the determinants of the scales is such as may enable 

 the insect to be either black or of a bright ruddy golden colour, and 

 the external stimulus which brings one or other of these extreme con- 

 ditions to tbe fore, appears to be that of temperature. 



If we apply the simplest elementary laws of vital force to our con- 

 sideration of the development of the pupa, we find that the following 

 facts hold good : — (1) The pupa, when first formed, has a certain 

 amount of inherent vital force, by means of which, both the process 

 of histolysis, and that of histogenesis, are carried on in it. (2) That 

 pupa which has the nearest approach to the normal amount of vital 

 force will undergo the most perfect histolysis and histogenesis, and 

 will produce an imago most nearly conforming to the natural type, 

 that is, to the form produced under the most healthy and satisfactory 

 conditions. Conversely, the pupa whose amount of vital force is 

 removed from the normal (whether by excess or defect) is the one in 

 which histolysis and histogenesis will be least perfect, and the imago 

 produced therefrom will be farthest removed from the normal type. 

 (3) That individual which has been best fed, and which has enjoyed 

 the most perfect health in the larval stage, will enter pupal life under 

 the most satisfactory conditions, and will (the pupal conditions being 

 equally satisfactory) emerge tberefrom as the best specialised product, 

 while the converse of this must also be true. 



Another important point appears also to depend on an elementary 

 principle. The vital force of the pupa is converted into energy ; the 

 energy at the disposal of the pupa is most probably directed, first, to 

 the building up of the vital and reproductive organs, afterwards to the 

 secondary organs or tissues, or such as are not necessary to life. There- 

 fore, any excess of energy in a pupa will be expended, as a rule, on 

 secondary structures rather than on vital ones, and so we find that a 

 weak or diseased pupa fails first in regard to non-vital tissues, such 

 as pigment, scales, wing membrane, etc. 



It would appear therefore that, as a general rule, pigment, scales, 

 etc., are well- or ill-developed in proportion to the amount of material 

 and energy available for the purpose. As a result, such insects as 

 pass through their metamorphoses at the normal temperature, pro- 

 duce the form which is normal for the district ; that is, they undergo 

 the normal processes of histolysis and histogenesis, and, in a state of 

 health, have at their disposal the energy requisite to give them the 

 normal wing-expanse, scaling and colour. If an increase or decrease 

 of temperature lowers the vitality of the pupa, it lessens the available 



