500 



INVERTEBRATE MORPHOLOGY. 



differ greater changes result, leading to hemimetabolism. This 

 occurs, for instance, in the Fish-flies (Ephemeridae) and 

 Dragon-flies (Libellula), in which the larvae are adapted for an 

 aquatic life and possess tracheal branchiae (Ephenieridse) and 

 other features which are lost, either gradually by successive 

 moults or suddenly at the last moult, the adult winged Dragon- 

 fly, for instance, issuing from a peculiar aquatic larva with 

 the merest rudiments of wings. 



Finally, a large number of forms are holometabolic. Ill 

 such cases the habits of the larvae are different from those of 

 the adults ; for instance, the larvae of the Butterflies, the 

 caterpillars (Fig. 231), are wormlike creatures with power- 



FIG. 231. LARVA, PUPA, AND IMAGO OF Pitris oleracea (from RILEY). 



ful jaws feeding on plant-tissues, while in the adults the mouth- 

 parts are adapted for sucking. The transformation from the 

 larva to the adult is accomplished by the intervention of a 

 resting stage or pupa, during which no nutrition is taken, and 

 when the transformation takes place the fully-developed 

 insect or imago issues from the ruptured skin of the pupa. 

 The pupa varies considerably in form in different groups, 

 in some being enclosed in a silken case manufactured by 

 the larva before the last moult and termed a cocoon. In 

 some cases the adult appendages project from the bod} r of 

 the pupa (pupa, libera), but in other cases they are united 

 with the surface of the body and but indistinctly visible 

 (pupa obtecta), an arrangement usually found in the Butter- 

 flies, whose pupae, owing to their frequent brilliant colora- 

 tion, are termed chryscdids. a term which has been somewhat 

 incorrectly extended to the mummy-like pupae of other forms. 



