TYPK TRACHEATA. 499 



approach of cold weather or on the exhaustion of the food- 

 supply males aud females appear by which true fertilized ova 

 are produced, and from these, surviving the winter, viviparous 

 heterogonous females develop. 



In the genus Phylloxera, which has played such havoc on grape-vines 

 in France, a greater complication of generations occurs. A winter egg, 

 which has survived beneath the bark of the vine, gives rise to wingless 

 forms which migrate to the roots, and there produce numerous genera- 

 tions. After a time winged forms appear which ascend from the roots, 

 and, reproducing parthenogenetically, increase rapidly in number and serve 

 to distribute the species over wider areas. Certain of these produce small 

 ova from which males develop, and others larger ones which give rise to 

 females, both sexes being destitute of both wings and digestive tract, and 

 by these forms the fertilized winter eggs are produced. 



In certain flies (Miastor, (Jecidomyia) psedogenesis occurs, 

 the female reproductive organs becoming mature while the 

 insect is still in the larval stage, and the ova, developing par- 

 thenogeuetically within the body, give rise to another gen- 

 eration of larvae. This process may be repeated several 

 times, the last generation of larvae developing into the adult 

 form (see Fig. 29). 



The more primitive Insects, the Thysanura and Collem- 

 bola, leave the egg in a form resembling the adult, differing 

 from it only in size and in the immaturity of the reproduc- 

 tive organs, and pass through no marked metamorphosis 

 during their post-embryonic development. Such forms are 

 termed ametabolic. A similar absence of metamorphosis is 

 found in certain forms degenerated by parasitism and lacking 

 wings, but these have evidently descended from winged forms 

 which passed through a certain amount of metamorphism, 

 so that the ametabolism is secondary and should be distin- 

 guished from the primitive ametabolism of the Thysanura. 

 In the majority of winged forms, however, a more or less 

 pronounced metamorphosis occurs. In the simpler cases the 

 young are distinguishable from the adults by the absence or 

 but slight development of the wings, which become larger 

 after successive moults, the adult form being thus gradually 

 acquired. In these cases of gradual metamorphosis the 

 habits of the adult and larva are similar, but where they 



