342 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



METAMORPHOSIS 



The first of the neopterous insects were closely related to stonefiies 

 and locusts and possessed incomplete metamorphosis, that is, they 

 passed through a series of nymphal stages Avhich gradually approached 

 the adult form. These are designated as the hemimetabolous Neop- 

 tera. The fourth step in insect evolution was the development of a 

 more complex type of metamorphosis, in which the insects pass 

 through a series of larval stages bearing little resemblance to the adult. 

 Eventually, they enter into one or two quiescent stages, during which 

 extensive morphological and physiological changes take place. These, 

 the holometabolous Neoptera, presumably had several advantages over 

 the hemimetabolous types. The immature forms, being very differ- 

 ent from the adults, could occupy dift'erent environments and feed on 

 different types of foods. The tissues of other organisms, both animal 

 and plant, were thus invaded by larval forms as internal parasites, 

 the adult insects remaining free-living and capable of flight. The 

 holometabolous insects make their first appearance in the Lower Per- 

 mian strata. The existence of two orders, the scorpionflies or Mecop- 

 tera and the Neuroptera in the Lower Permian, shows that complete 

 metamoiphosis must have begun before the end of the Upper Car- 

 boniferous period. Starting from the beginning of the Permian, 

 when only about 5 percent of the known species of insects had complete 

 metamorphosis, the percentage of species has progressively increased 

 to the present maximum of 88 percent. 



A simple phylogenetic diagram, shown in figure 1, superimposed on 

 the geological time scale, serves to summarize this general aspect of 

 insect evolution. The three modifications — origin of wings, wing 

 flexing, and complete metamorphosis — mark the points of separation 

 of the phylogenetic lines. Since holometabolous insects are known to 

 have existed from the Lower Permian strata, the upper phjdogenetic 

 division must have taken place before the end of the LTpper Carbonif- 

 erous ; and since neopterous insects are known from the lowest of the 

 Upper Carboniferous strata, the middle division, or wing flexing, must 

 have taken place in the Lower Carboniferous, which is beyond the 

 present record of the insects. The first phylogenetic division nuist 

 have occurred even earlier. 



FOSSIL RECORD 



Turning from this phylogenetic treatment of the insects, I shall next 

 consider their history as it is now actually known from the fossil rec- 

 ord. This discussion will involve some mention of extinct orders and 

 an explanation of my point of view on this controversial subject. 

 The artificial and arbitrary nature of higher taxonomic categories is 



