504 



we can see now the imago, whose development has been so long 

 delayed, and which would have appeared in this state in the 

 egg if growth of the imago could have been possible. This 

 "larval" moult in the different orders takes place when the 

 embryo is in various states of development. In Lepidoptera 

 the appendages are only feebly developed; the hard secretion 

 of the exposed epithelium effectually serves to hide these. 

 In the cbalcids appendages are always well developed ; indeed, 

 in this group it is possible to say that the period preceding 

 the pupal moult is the period of growth in size of the external 

 form of the developing imago, the pupal period the period of 

 its differentiation. This generalization cannot, however, be 

 applied to the internal organs; it serves merely to emphasize 

 the fact that the peculiar organization of the external features 

 of insects have mainly been responsible for the evolution of 

 metamorphosis. It is scarcely necessary to remark, that the 

 view so often held, viz., that metamorphosis commences in 

 the pupal period, is quite erroneous ; for some few tissues this 

 is true, but more usually the most profound changes occur 

 under the shelter of the old larval cuticle. 



It is interesting, in conclusion, to observe that the insects 

 do not, after all, provide an exception to the generalization 

 of Von Baer, that resemblances between different species 

 become closer as we examine ever earlier stages of their 

 embryonic development. It has more than once been remarked 

 that the pupae of related insects are more alike than their 

 larvae; but when we remember that the larva is a younger 

 product of evolution than the embryo which succeeds it in 

 development, and which is revealed at the pupal moult, then 

 our confidence in the law, the truth of which has lately been 

 so much questioned, must become stronger than ever. 



SUMMARY. 



A. External Features. 



The subject of this investigation is a small chalcid wasp, 

 Nasonia, parasitic on muscid pupae, and world-wide in dis- 

 tribution. 



The larva, which is composed of fifteen segments (2 head, 

 3 thoracic, 10 abdominal), feeds for three days and then begins 

 to metamorphose; a day later the contents of the intestine 

 are voided (defaecation period), another day later, after the 

 developing imaginal discs have grown into the form of the 

 imago, the "larva" moults and discloses the pupa. Further 

 development of imaginal organs and disintegration of larval 



