REMINGTON: THE "APTERYGOTA" 499 



11. Malpighian Tubules: These are apparently absent in the Collembola and Protura, 

 but present in all of the seven other groups. 



12. Head Folds: In the Collembola, Protura, and Entotrophi there are lateral out- 

 growths of the head capsule which fuse with the labium (second maxillae) and enclose 

 all but the tips of the mandibles and first maxillae. None of the other groups are thus 

 "entotrophous." The phylogenetic significance of this condition is obscure, but there seem 

 to be excellent reasons for regarding its origin as independent in each of these three 

 groups. 



13. Cerci: These are present in primitive Ptevygota, Thysanura, Entotrophi, and 

 Symphyla, but absent in Protura, Collembola, Chilopoda, Diplopoda, and Pauropoda. The 

 similarity between cerci of Symphyla and some Entotrophi (Projapygidae) has been 

 regarded as a phylogenetic indicator, but they are enough different so that little em- 

 phasis can be placed on them. The similarities of eversible vesicles and certain styli 

 among the Protura, Symphyla, Entotrophi, and Thysanura seem to be of greater im- 

 portance. 



14. Other Atdominal A^rpendages: The midventral collophore of the fourth post- 

 cephalic segment is always present in the Collembola and never present in any other 

 group. Its suggested homology with eversible vesicles of Entotrophi and Thysanura is 

 without evidence. The furcula, or spring, usually present on Collembola, has no reason- 

 ably clear homology to any appendage known outside the Collembola. If, as has been 

 suggested by Imms and others, the Collembola evolved by paedomorphosis from hexa- 

 podous first instar nymphs of a myriapod-like ancestor, then any abdominal appendages 

 like the furcula and retinaculum must be of new origin without leg homologies in other 

 Arthropoda. The only other notable abdominal appendage is the median caudal appendage 

 of the Thysanura. It is probably the fourteenth postcephalic tergite and may be homolo- 

 gous with a similar appendage in Ephemerida. 



15. Germarium: In the Protura, Entotrophi, Thysanura, Pterygota, and some Pauro- 

 poda the germarium is apical; in Collembola, Symphyla, the remaining Pauropoda, and 

 probably the Diplopoda and Chilopoda the germaria are lateral or scattered. 



16. Phallus: Males of the Thysanura and the unspecialized Pterygota have a dis- 

 tinctive phallic structure with no phallic homologue in any of the seven other groups. 

 The phallus of the Pterygota presumably originated in a protothysanuran, and was first 

 modified for intromission in conjunction with female structures in a protopterygote. 

 Intromittent organs of a radically different kind are found in males of the Pauropoda, 

 Diplopoda, Protura, and Symphyla. The "penis" of Chilopoda is more like that of the 

 Pterygota and Thysanura, but it seems not to be homologous. Recent papers have shown 

 that Collembola and Thysanura do not have intromissive copulation. The details of the 

 exchange of sperm are apparently not known for the Pauropoda, Symphyla, Protura, and 

 Entotrophi. 



17. Ecdysis: The Pterygota without exception have a physiological mechanism which 

 arrests ecdysis after sexual maturity is reached (the pre-imaginal molt of Ephemerida 

 does not contradict this generalization). In all the eight other groups ecdysis continues 

 after reproduction begins, with the possible exception of the Pauropoda, where Tiegs 

 found no evidence of imaginal molting in the one species he studied. 



My view, after several years of self-debate and study of the many discourses 

 in print concerning the relationships of these groups, is that the most justi- 

 fiable course is not to exclude the Collembola, Protura, and perhaps Entotrophi 

 from the Insecta, but rather to extend the Insecta, almost in the Linnaean sense, 

 to include all nine of these groups in an arrangement approaching the following : 



Subphylum Insecta 

 Section Myocerata 



Superclass Dignatha 



Class Pauropoda 



Class Diplopoda 



Superclass Trignatha 



Class Chilopoda 



