184 THE entomologist's record. 



(3) The colour of the membrane of the wing. The inter-relationship of 

 these three factors must be considered, if we are to get any true idea of 

 the changes which the colours of insects undergo when exposed to ab- 

 normal temperatures. In Mr. Merrifield's experiments, referred to by 

 Prof. Weismann, and in the natural variation which occurs in Southern 

 Europe, there can be no doubt that all three are affected. 



If we apply the simplest elementary laws of vital force to the pupa, we 

 shall (as I have previously pointed out in the pages of this magazine), 

 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 " rehabilitation " 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 

 "rehabilitation," 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 farthest removed fi'oni the normal (whether by excess or defect) 

 is the one in which "histolysis" and "rehabilitation" 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 

 pujjal life under the most satisfactory conditions, and will (the pupal 

 conditions being equally satisfactory), emerge therefrom as the best 

 specialised product, whilst the converse of this must also be true. 



Another important point appears also to depend on an elementary 

 principle. The vital force of the puj^a 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 di.-eased j^upa fails first in regard to non-vital tissues, such as 

 pigment, scales, wing membrane, etc. 



It woiild 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 change at the normal teniperatm-e produce the form 

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

 processes of histolysis and rehabilitation, 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 piipa, it lessens the available 

 energy. The insect, therefore, does not develop under such favourable 

 conditions ; it needs what energy it possesses to build up its vital 

 organs, and so fails in jDerfectly building up the secondary tissues. 

 This failure is in direct proportion to the degi'ee in which the vitality 

 is lessened. If the temperature during the period of active development 

 be below a certain degree, the vital force ceases to act at all and death 

 results. Heat greater than that to which tlie insect is normally 

 subjected, instead of lowering the vitality to the lowest ebb at which 

 life can be sustained, affects the hisiolysis and rehabilitation in a 

 directly opposite manner. Under its influence the vital processes are 

 carried on at express speed. Energy is expended at the fastest rate 



