108 



THE EEPOET OF THE 



No. 36 



siicli a view was promulgated, for such a study clearly indicates that the lines 

 of development lead from the common ancestors of the isopods, Tanaidacea, 

 Cumacea, and other Crustacea, through those of the Machilidae and Lepismatidae 

 to the ancestors of the most primitive representatives of the winged insects such 

 as the mayflies (Ephemerida) and stoneflies (Plecoptera). The structural re- 

 semblance between the mayflies and the Machilidae, or that between the Plecoptera 

 and the Lepismatidae, is most striking, and the lines of descent of the Machilidae 

 and Lepismatidae clearly lead back to Crustacea-like, rather than to " Myriopod "- 

 like ancestors. It must be admitted, however, that certain other apterygotan 

 insects such as the Campodeidae, Protura, etc., are extremely closely related to 

 certain " Myriopoda " such as Scolopendrella, Pauropus, etc., but the lines of 

 descent of these forms appear to represent merely side issues of the main trunk 

 which leads to the evolution of the pterygotan insects (unless such insects as 



HIGHER. ORDERS 



PSOCIDA 



ZORAPTERA 



ISOPTERA 



BLATTIDA 

 EPHEMERIDA 

 MACHILID 



COLEOPTERA 



DERMAPTERA 

 MBIIDA 

 PLECOPTERA 



LEPISMATIDA 



6 



Campodea, Japyx, and other insects of the apterygotan order Iihabdura, are near 

 the forms giving rise to the line of development of the pterygotan order Dermap- 

 tera, as I formerly held to be the case — but a further study of the insects in 

 question has tended to discredit this view). 



Although the main lines of descent of the pterygotan insects appear to avoid 

 the " myriopodan " side of the ancestry of insects and to lead back more directly 

 to Crustacea-like forms through ancestors resembling the Machilidae and Lepisma- 

 tidae, the dual relationship of apterygotan insects to the " Myriopoda " as well as 

 to the Crustacea, cannot be ignored. This dual relationship is expressed graphic- 

 allv in Eig.l. As is shown in the figure, the lines of descent of the " Myriopoda," 

 Tnsecta, and higher Crustacea (Isopoda, Tanaidacea, Cumacea, etc.) taken at the 

 level "A," are quite distinct (as is represented by cross sections of these lines of 

 descent shown in Fig. 2) ; but at the level " B," where the lines of descent begin 

 to converge as they approach their common source, it is evident that the members 

 of the three groups come very close together, and those insects occupying the 



