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DEVELOPMENT 159 



Significance of Metamorphosis. " The essential features of meta- 

 morphosis/' says Sharp, "appear to be the separation in time of growth 

 and development and the limitation of the reproductive processes to a 

 short period at the end of the individual life." 



The simplest insects, Thysanura, have no metamorphosis, and show 

 no traces of ever having had one. Hence it is inferred that the first 

 insects had none; in other words, the phenomenon of metamorphosis 

 originated later than insects themselves. Successive stages in the 

 evolution of metamorphosis are illustrated in the various orders of 

 insects. 



The distinctive mark of the simplest metamorphosis, as in Orthop- 

 tera and Hemiptera, is the acquisition of wings; growth and sexual 

 development proceeding essentially as in the non-metamorphic insects 

 (Thysanura and Collembola) . Here the development of wings does not 

 interfere with the activity of the insect; its food habits remain unaltered; 

 throughout life the environment of the individual is practically the same. 

 Even when considerable difference exists between the nymphal and 

 imaginal environments, as in Ephemerida and Odonata, the activity of 

 the individual may still be continuous, even if somewhat lessened as the 

 period of transformation approaches. 



With Neuroptera, the pupal stage appears. In these and all other 

 holometabolous insects the larva accumulates a surplus of nutriment 

 sufficient for the further development, which becomes condensed into 

 a single pupal stage, during which external activity ceases temporarily. 



With the increasing contrast between the organization of the larva 

 and that of the imago, the pupal stage gradually becomes a necessity. 

 Metamorphosis now means more than the mere acquisition of wings, for 

 the larva and the imago have become adapted to widely different en- 

 vironments, chiefly as regards food. The caterpillar has biting mouth 

 parts for eating leaves, while the adult has sucking organs for obtaining 

 liquid nourishment; the maggot, surrounded by food that may be ob- 

 tained almost without exertion, has but minimum sensory and locomotor 

 powers and for mouth parts only a pair of simple jaws; as contrasted 

 with the fly, which has wings, highly developed mouth parts and sense 

 organs, and many other adaptations for an environment which is 

 strikingly unlike that of the larva; so also in the case of the higher 

 Hymenoptera, where maternal or family care is responsible for the help- 

 less condition of the larva. 



Thus it is evident that the change from larval to imaginal adapta- 

 tions is no longer congruous with continuous external activity; a quies- 



