No. 625] THE EVOLUTION OF ARTHROPODS 



175 



The jSTeuropteron group comprises the Neuroptera, 

 Trichoptera, and Mecoptera, with their immediate rela- 

 tives. They and their descendants are very closely re- 

 lated to the members of the psocid group mentioned 

 above, and the two lines soon merge in a common ances- 

 try when traced back toward the plecopteroid stem. The 

 Neuroptera seem to be a very ancient type, and have 

 inherited certain primitive characters which would indi- 

 cate that their line of development branched off at a com- 

 paratively low level. Both the Trichoptera and the Me- 

 coptera are descended from ancestors quite like the pres- 

 ent-day Neuroptera, while the Lepidoptera branched off 

 near the trichopteron line of descent, and the Diptera 

 branched off near the mecopteron line (see also Cramp- 

 ton, 1917&). The Siphonaptera were apparently de- 

 scended from ancestors not unlike phorid Diptera. 



The Hymenoptera represent a somewhat aberrant 

 group having affinities with both the members of the 

 psocid and neuropteron groups. Their line of descent 

 probably originated near the point at which the psocids 

 and Neuroptera branched off, and they inherited many 

 features also present in the members of both of these 

 groups, so that their line of development must have ac- 

 companied or extended beside those of the other two for 

 a oonsidcrablo distance before it branched off to follow 



The i^oints which should be especially emphasized in 

 regard to the evolution of the insectan branch of the 

 arthropod lines of development may be briefly summa- 

 rized as follows : 



The ancestors of artliropods were not of any one type, 

 but varied in regard to tlie number of segments compos- 

 ing tlieir bodies, tlio outline of tlie body, etc.: and while 

 some (.r til. •Ill may lia\-(' Lccii as small as tlic tardigrades, 

 it is 111. .IV |ir,, liable that llic typ.'s woul.l in' iticluded be- 

 tween the extremes represented l)y the ( )ny('liophora and 



