146 



ENTOMOLOGY 



Equally anomalous are the changes undergone by Polynema, a proc- 

 totrypid 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 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." 



mo 



- m 



..-m 



FIG. 219. Stages in the hypermetamorphosis of Platygaster, A, first larva; B, second 

 larva; C, third larva; a, antenna; b, brain; /, fat-tissue; h, hind intestine; m, mandible; 

 mo, mouth; MS, muscle; n, nerve cord; r, reproductive organ of one side; s, salivary gland; 

 I, trachea. After GANIN. 



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 evolu- 

 tion 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. 



