v] TRANSFORMATIONS 45 



rapid but continuous changes, which, during the early 

 pupal period, convert the elongate nerve-cord of the 

 caterpillar with its relatively far-separated ganglia 

 into the shortened, condensed nerve-cord of the Tor- 

 toise-shell butterfly (Vanessa urticae) with several 

 of the ganglia coalesced. In many Diptera, on the 

 other hand, the nervous system of the larva is more 

 concentrated than that of the imago. 



The tubular heart also of a winged insect is the 

 directly modified survival of the larval heart. 



Similarly the reproductive organs undergo a gra- 

 dual, continuous development throughout an insect's 

 life-story. Their rudiments appear in the embryo, 

 often at a very early stage ; they are recognisable in 

 the larva, and the matured structures in the imago 

 are the result of their slow process of growth, the 

 details of which must be reckoned beyond the scope 

 of this book. For a full summary of the subject 

 the reader is referred to L. F. Henneguy's work (1904) 

 containing references to much important modern 

 literature, which cannot be mentioned here. 



On the other hand, the digestive system of insects 

 that undergo a metamorphosis, passes through a 

 profound crisis of dissolution and rebuilding. This 

 is not surprising when we remember that there is 

 often a great difference between larva and imago in 

 the nature of the food. The digestive canal of a 

 caterpillar runs a fairly straight course through the 



