IV. INSECTA: HEXAPODA 



415 



anlagen of the wings as small folds in the chitinous coat of the meso- and 

 metathorax; these increase with each successive molt, until, with the last, 

 they become functional wings in size, form and motion. Inside of each 

 wing pad (B, i and 2) there is the anlage of the wing of the next stage. 

 Since the larvae, from lack of wings, are forced to live under different 

 conditions from the adults, conditions which demand special structures, 

 the differences between the larva? and the adults are emphasized by the 

 presence of specific larval organs. Thus the aquatic larva: of dragon- 

 flies and Mayflies, are distinguished, not only by the absence of wings, 

 but by different form, different shaped mouth parts, and especially by 

 the tracheal gills (fig. 452), usually lost at the last molt. 



Increase in larval characters leads to complete metamorphosis. In 

 order to profit as much as possible by its adaptation to its environment 

 the larva retains its shape as long as possible; the gradual change to the 

 adult is suppressed and the alteration in form is postponed until the end 

 of the larval life, to the period between the last two molts. In this inter- 

 val there is such an energetic transformation of the organism that ordi- 

 nary vital functions, especially motion and feeding, are interfered with or 

 rendered impossible. This last stage therefore becomes a period of rest 



FIG. 458. Larva and free pupa of May beetle, a', a", fore and hind wings; an, anus; 

 at, antenna;; o, eyes; p'-p'", legs; st, spiracles. 



the pupal stage, the existence of which is important in the definition of com- 

 plete metamorphosis. The more complete the condition of rest the more 

 pronounced is the holometabolous development. From this point of view dif- 

 ferent types of pupa? are distinguished: pupa? liberae, pupa? obtecfce, and 

 pupa? coarctata?. In a free pupa (pupa liber a) the appendages stand out 

 from the body (fig. 458), so that not only the segmentation of the body but 

 the antenna?, legs, wings, and often the mouth parts of the imago are visible. 

 Such pupa? have a certain power of motion, as, for instance, the pupa? of 

 many Neuroptera and mosquitos, the latter rising and falling in the water. 

 The covered pupa? (pupa: obtectff) at the moment of pupation have free 

 appendages which with the hardening of the chitin become closely 



