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An equally interesting question is the relation in which 

 the insects which show a metamorphosis stand to those in 

 which it is absent; this question has been discussed by Lub- 

 bock (1874), and more recently by Deegener (1909), Lub- 

 bock's conclusion, that the metamorphosis was made necessary 

 by the larvae developing different feeding habits and con- 

 sequently different mouth-parte from those of the adult insects, 

 is not very satisfactory. While it is true that the transition 

 from one to the other would have to be slow and would have 

 to take place during a resting stage, it fails to account for 

 the metamorphosis of structures of almost negligible import- 

 ance, such, for example, as the fine somatopleural membrane 

 beneath the integument. It fails also to explain the meta- 

 morphosis of the feeding organs in insects in which the larvae 

 and adults have the same feeding habits, such, for example, 

 as many of the carnivorous and leaf -eating beetles. More- 

 over, the real thing to show is why the larval form should 

 ever have been evolved, necessitating the parallel evolution 

 of a metamorphosis, when some insects, very successful in 

 the struggle for existence, have got on so well without it. 

 The conclusion of Deegener, that the larval form is a stage 

 gradually inserted between the early embryo state and the adult, 

 is undoubtedly quite correct, and seems to be usually accepted 

 to-day. Nevertheless he throws no light on the reason why 

 such a form should ever have been evolved, nor does he 

 explain why it later transforms itself into the mature insect. 



It was to answer these several questions that the present 

 work was undertaken. The insect which I have employed is 

 a small chalcid wasp, Nasonia hrevicornis, very common in 

 Australia and America as a parasite on exposed pupae of 

 muscid flies. According to Mr, A. A. Girault it is identical 

 with Nasonia ahnormis, Boheman, from Europe, and is evi- 

 dently of world-wide distribution. As the work proceeded 

 I found myself at a disadvantage in that very little was 

 known about the internal anatomy of chalcid wasps, while 

 the study of the anatomy of the larvae had also been 

 greatly neglected, and more than one very serious 

 misinterpretation have been accepted as fact. I have 

 therefore resolved to extend the scope of the paper. In 

 the first portion the various organs of the larva 

 and adult are described and a fairly detailed account of 

 them is given as they transform from the larval to the adult 

 conditions. In the second part I shall attempt to explain 

 the physiological basis of the metamorphosis, and to discuss 

 the factors which have underlain the evolution of the process. 



The earlier parts of this investigation were carried out 

 in the Laboratory of the Biology Department, University of 



