ENTOGNATHY IN APTERYGOTES — TUXEN 413 



bles. On the other hand the musculature of the maxillae is distinctly 

 in accordance with that of pterygote insects, the tentorium having 

 taken over the role of the fulcrum. There are also likenesses to the 

 maxillae in Symphyla (Tiegs, 1940, p. 164), but in no other myri- 

 apodan group. 



The symplesiomorph characters thus suggest relationship to both 

 Myriapoda and Insecta. As to the synapomorph characters, it must 

 be a matter of opinion whether one will regard them as indicating rela- 

 tionship, but it seems to me that the similarity in the way entognathy 

 is brought about and the shape it has taken in the three groups is so 

 great that it must involve a monophyletic origin. The shape of the 

 gnathal pouches, the shape of the fulcrum, and the shape also of the 

 mandibles and maxillae — the former suggesting Myriapoda, the latter 

 Insecta and Symphyla — are so similar in the three groups that only a 

 most curious coincidence would cause these characters to appear 

 simultaneously in three independent cases. 



I think, therefore, that the relationship between the three groups 

 of entognathous apterygotes is much closer than between them and 

 any other group, and quite certainly closer than between Diplura and 

 Thysanura, or between Diplura and Symphyla. So, if entomologists 

 want to remove Collembola and Protura from Insecta they must take 

 Diplura with them ; one cannot find in the Diplura any link to the 

 "Insecta" and therefore neither do they support the theory of the 

 symphylan origin of insects. To me it seems that all three groups 

 taken as a unit have a distinct relationship to both Myriapoda and to 

 Insecta, but form a branch of their own parallel to these ; because of 

 their hexapodan condition they may well be treated in entomological 

 textbooks, but they do not represent "primitive insects" and they 

 should not be treated together with Thysanura as "primitively wing- 

 less insects," because their relation to Thysanura is not greater than 

 to many other arthropod groups. Perhaps it would be most con- 

 venient to follow Hennig (1953) and treat them together as a class 

 of their own: Entognatha. 



ABBREVIATIONS USED ON THE FIGURES 



ant, antenna. gnP, gnathal pouch. 



cd, cardo. hph, hypopharynx. 



dA, so-called dorsal arms. lb, labium. 



eph, epipharynx. Ibr, labrum. 



fi, filamento di sostegno. Ic, lacinia. 



fu, fulcrum. lin, lingua. 



ga, galea. mcR, midcranial ridge. 



