Insects and their world 



Fig. 3. A stone-fly. Order Plecoptera 



though now without wings, seem to be related to the Diptera, or true 

 flies, and traces of wings have been reported in some pupae. 

 The Sub-class Pterygota falls again into two major divisions : 



Division I: Exopterygota (or Hemimetabola) 



These insects hatch from the egg in a form already very much hke 

 that of the adult, except for the absence of wings. The wings make their 

 first appearance as mere pads, which are visible externally, but they do 

 not begin to function as wings until after the final moult into the adult 

 (or imago). The Mayflies (Order Ephemeroptera) are exceptional in 

 having wings in the stage immediately before the adult, the subimago. 



The name Exopterygota refers to the externally visible wing-pads, and 

 the alternative name Hemimetabola to the gradual and continuous trans- 

 formation towards the adult, in contrast to the abrupt metamorphosis 

 of inseas in the other Division. 



The immature form of such a hemimetabolous insect is called a 

 nymph. Some entomologists object to this term, either because of pos- 

 sible confusion with the French 'nymphe\ which means a pupa (see 

 below), or because they consider that no clear line can be drawn 

 between this type of development and the next one ; but no convenient 

 alternative has been found. 



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