Dec, 1916.] Relationships of Apterygotan Insects. 



285 



branched off at a comparatively early stage of development (see 

 fig. I). 



The " Rhabdiiran " insects with their concealed mouthparts, vesti- 

 geal first abdominal legs, etc., appear to approach the Proturan forms 

 on the one hand, and also lead up to the Machilis-Vike forms w^ith 

 well-developed cerci, styli, etc., on the other : so that they might be 

 regarded as somewhat intermediate between the two groups, although 

 their closest affinities are with the Thysanuran group (i. e., Machilis, 

 Lcpisma, etc.). Such forms as Anajapyx serve to connect the 

 Campodeidse with the Japygidse. 



The Thysanuran group {Machilis, Lepisma, Nicoletia, etc.) ap- 

 proaches quite closely to the lower Pterygotan forms, the Plecoptera, 

 Blattidae, and Ephemeridae being as nearly related to them as any of 

 the Pterygota. 



LEPISM4T0II1E3 

 UACHILOIDES 



CAUPODEOIDBS 

 JAPYGOIDES 



PTEBYGOTA 



CRUSTACEA 



APTERYGOTA 

 IJYBIOPODA 



EOSENTQUOIHES 



/EOTOMOBRYOIDES 

 SMINTHUROIDES ONYCHOPHORA 

 UMELIDA 



Fig. I 



ARACHNIDA 

 MEROSTOMATA 



TRILOBITA 



The relationships of the principal Apterygotan groups are repre- 

 sented in fig. I, while in fig. 2 the lines of descent of the more im- 

 portant groups related to the Insecta have been shown. It is im- 

 possible to represent the rather complicated interrelationships of the 

 different groups in a figure drawn in one plane, since the various 

 lines of descent approach one another from different angles, and it 

 is practically impossible to represent this correctly without making 

 such an intricate crossing of lines as to render the diagram almost 

 unintelligible, and therefore useless. On this account, the discussion 

 given above rather than the diagram, should be taken as setting forth 

 the true relationships of the forms here described. 



