PUPAL DEVELOPMENT AND COLOUR IN IMAGO. 313 



produced Avill be farthest removed from the normal type. (3) That 

 individual wliich 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 jjupal conditions being equally 

 satisfactory) emerge therefrom as the best specialised product, whilst 

 the converse to this must also be true. 



The second point also deals with an elementary principle. The 

 vital force in 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 reproductive organs, and afterwards to the secondary 

 organs or tissues, or such as are not necessary to life. Therefore, an 

 excess of energy in a pupa will be expended as a rule on secondary 

 structures rather than on vital ones, and we find that a weak or 

 diseased pupa fails first in regard to non-vital tissues, such as pigment, 

 scales, wing membrane, ifec. 



The females of insects, as compared with the males, require an 

 excess of energy for those structures necessary to the reproduction of 

 the species ; they, therefore, have a smaller surplus to devote to the 

 formation of the non-vital tissues, and, as we Avell know, frequently 

 fail very markedly in their development of these. 



We are now in a position to understand that, as a general rule, 

 pigment, scales, &c. are produced in proportion to the amount of 

 material and energy available for the purpose. 



These and other general principles have to be considered when we 

 attempt to discuss the results which Mr. Merrifield produces by his 

 temj^erature experiments. To attempt to discuss the results without 

 first understanding these principles can only lead to futile con- 

 clusions. 



If we now apply these principles what do we find ? Insects Avhich 

 are allowed to pass through their changes at the normal temperature 

 produce the form Avhich is normal for the district ; that is, they 

 undergo the normal processes of histolysis and of rehabilitation, and in 

 A state of health have at their disposal the energy retpiisite to give 

 them their ordinary wing expanse, scaling and colour. Now what 

 does Mr. Merrifield do in his exjjeriments. He subjects the pupa to a 

 low temperature. This of necessity lowers the vitality of tlie pupa, 

 and so lessens the available energy. The insect, therefore, does not 

 develop under normal conditions, and an abnormality is the result. 

 The insect must use what energy it has to build up its vital organs, 

 and fails in building up perfectly its secondary tissues — colour, scales, 

 wing membrane, and fails, too, in direct projiortion to the degree in which 

 the vitality is lessened. Below a certain temperature during the jDcriod 

 of active development the vital force ceases to act at all, and the result 

 is death. Heat, greater than that to which the insect is normally 

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

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

 directly opposite manner. Under its influence the vital processes 

 ai'e carried on at express speed. Energy is expended at the fastest 

 rate possible, and the tissues are formed Avithout having sufficient 

 time to mature as they would under normal conditions (we may here 

 sujjpose these to be those which are most beneficial to the species, and 

 probably as being the result of "natural selection"), the surplus material 

 is rapidly utilised, with the result that as marked an abnormality is 



