20 



The Nature of Metamorphosis. 



By J. W. TuTT, F.E.S. Read March ^th, 1899. 



It is well known to all entomologists that the embryo of an insect 

 passes through various changes in form before it hatches from the 

 egg, and that after hatching other changes almost as remarkable 

 take place, especially in those insects with a complete metamor- 

 phosis, each change essentially being an adaptation to, and fitting the 

 insect for, a particular mode of life. 



The life of the holometabolic insects is such that it may be con- 

 veniently divided into four distinct periods or stages — the egg, larval, 

 pupal and imaginal stages. The term metamorphosis is applied to 

 the post-embryonic changes, /. e. to those that take place after the 

 larva has hatched from the egg. All holometabolic insects live, as it 

 were, three distinct and different lives, in each of which its modes of 

 life, of nutrition, and environment are entirely dissimilar. Hence its 

 habits being different, it has to meet its enemies and to protect itself 

 in various ways. The caterpillar is essentially a different animal 

 from the pupa, and the pupa from the imago. Organs that are well- 

 developed in the one stage may become obsolete in the succeeding 

 ones, whilst ill-developed organs may become well-developed in later 

 ones. I'he modifications of organs extend to their functions, and 

 hence we find the habits possible to one stage quite impossible in the 

 others. 



In the Synaptera, development is direct, the young differing neither 

 in form, structure, nor habits, from the adult. The primary form and 

 general appearance of the animal is maintained throughout its exist- 

 ence, there is not even a tendency to a partial metamorphosis exhibited, 

 and the insects are consequently termed Ametabola. This suggests 

 strongly that metamorphosis took place after and not simultaneously 

 with the first appearance of insects, and further that it was a pheno- 

 menon that was induced after the first winged insects appeared. 



The Heterometabola and Holometabola comprise the winged 

 insects. To the first group are relegated those insects in which the 

 adult differs mainly from the newly-hatched young in having wings, 

 and in being without an inactive resting or pupal stage. The wings 

 of these insects are acquired after successive moults, the insect 

 remaining active and seeking its prey in all its stages. The Hetero- 

 metabola comprise the Orthoptera, Dermaptera, Platyptera, Ephe- 

 merida, Odonata, Thysanoptera and Hemiptera (except male Coccids, 

 which have a quiescent stage preceding the winged form). 



