v] TRANSFORMATIONS 41 



from the one to the other is not easy to explain. 

 Sharp has expressed the divergence by the terms 

 Endopterygota, applied to all the orders of insects 

 with hidden wing-rudiments (the 'Metabola' or 

 1 Holometabola ' of most classifications) and Exo- 

 pterygota, including all those insects whose wing- 

 rudiments are visible throughout growth ('Hemi- 

 metabola ' and ' Ametabola '). Those curious lowly 

 insects, belonging to the two orders of the Collembola 

 and Thysanura, none of whose members ever develop 

 wings at all, form a third sub-class the Apterygota 

 (see Classificatory Table, p. 122). 



Not the wings only, but other structures of the 

 imago, varying in extent in different orders, are 

 formed from the imaginal discs. For example, de 

 Reaumur and G. Newport (1839) found that if the 

 thoracic leg of a late-stage caterpillar were cut off, 

 the corresponding leg of the resulting butterfly would 

 still be developed, although in a truncated condition. 

 Gonin has shown that in the Cabbage White butterfly 

 (Pieris brassicae) the legs of the imago are repre- 

 sented, through the greater part of larval life, only by 

 small groups of cells situated within the bases of the 

 larval legs. After the third moult these imaginal 

 discs grow rapidly and the proximal portion of each, 

 destined to develop into the thigh and shin of the 

 butterfly's leg, sinks into a depression at the side 

 of the thorax, while the tip of the shin and the 



