DEVELOPMENT 1 77 



From now on, the development is essentially like that of other 

 parasitic Hymenoptera. 



Equally anomalous are the changes undergone by Poly- 

 nesia, a proctotrypid parasite in the eggs of dragon flies, and 

 by the proctotrypid Teleas, which affects the eggs of the tree 

 cricket (CEcanthus). In all these cases the larvae go through 

 changes which in most other insects are confined to the egg 

 stage. In other words, the larva hatches before its embryonic 

 development is completed, so to speak. 



Significance of Metamorphosis. " The essential features 

 of metamorphosis," 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 indi- 

 vidual life." 



The simplest insects, Thysanura, have no metamorphosis, 

 and show no traces of ever having had one. Hence it is in- 

 ferred that the first insects had none ; in other words, the phe- 

 nomenon of metamorphosis originated later than insects them- 

 selves. Successive stages in the evolution of metamorphosis 

 are illustrated in the various orders of insects. 



The distinctive mark of the simplest metamorphosis, as in 

 Orthoptera 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 nym- 

 phal and imaginal environments, as in Ephemerida and 

 Odonata, the activity of the individual may still be continu- 

 ous, even if somewhat lessened as the period of transforma- 

 tion approaches. 



With Neuroptera, the pupal stage appears. In these and 

 all other holometabolous insects the larva accumulates a sur- 

 plus of nutriment sufficient for the further development, which 

 becomes condensed into a single pupal stage, during which 

 external activity ceases temporarily. 

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