45 



difference in the relative quantities of the materials composing the 

 pigment. Many ill-pigmented specimens of Lepidoptera, probably 

 resulting from this cause, frequently come under one's notice. When, 

 however, naturalists ascribe the origin and modification of pattern to 

 these internal forces, we disagree most strongly ; for, whilst the forma- 

 tive material of the patterns is undoubtedly the result of internal 

 activities, the arrangement of the material is, in my opinion, and as I 

 have elsewhere discussed at length, due to utility, and has been 

 brought about and perfected by natural selection. This, of course, 

 brings us to the threshold of the absorbingly interesting subjects of 

 mimicry and protective resemblance — matters quite outside the possi- 

 bility of consideration in so short a paper. 



One other aspect of our subject, however, has been much studied 

 of recent years, and can hardly be altogether passed over in silence. 

 I refer to the phenomena attending the " metamorphosis of insects." 

 Striking as are the changes of form through which the embryo passes 

 before leaving the egg-shell, the changes that these insects, with a 

 complete metamorphosis, subsequently undergo are almost equally 

 remarkable. The cause of these changes is undoubtedly to enable 

 them to lead a different 'mode of life, and to adapt them to the 

 changed conditions that characterise the mode of life adopted at 

 various stages of their existence. 



The post-embryonic life of a winged insect, such as a fly, bee, &c, 

 is divisible into three stages — the larva, pupa, and imago, the term 

 metamorphosis being applied to the changes that take place during 

 the post-embryonic stages of life. The larva of such an insect (fly, 

 bee, butterfly, beetle) lives an entirely different life from that led by 

 the pupa of the same animal, and this again differs from that led by 

 the imago or perfect insect ; so that the insect in its three forms leads 

 to all intents and purposes three distinct lives, existing under entirely 

 dissimilar surroundings, and necessitating entirely different habits. 

 To enable it to do this the organs themselves have to undergo con- 

 siderable modification. Certain structures useful in one stage may 

 be useless in others, and hence disappear (e. g. the lepidopterous 

 prolegs) ; others are modified entirely in character, owing to different 

 requirements in the mode of use (e. g. the mouth parts) ; again, others 

 arise, as it were, suddenly from structures previously in a very rudi- 

 mentary state (e.g. antennas and wings). The changes that take place 

 at a metamorphosis occur not only in the external body and its append- 

 ages, but also in the internal organs, the changes extending some- 

 times not only to the shape and general external character of the 

 organs, but also to a difference in function. 



All insects do not undergo the same degree of metamorphosis. 

 The differences between the larval, pupal, and imaginal condition of 

 Hymenoptera, Coleoptera, Diptera, Lepidoptera, and Trichoptera are 

 very marked ; but to the early stages of the Heterometabola — Orthop- 

 tera, Hemiptera, Odonata, &c. — the term larva and pupa are scarcely 

 applicable. Many naturalists use the term "larva" for the early 



