REMINGTON: THE "APTERYGOTA" 497 



Handlirsch (1903) followed Grassi's separation, but he elevated Grassi's sub- 

 orders to the rank of classes, and the families became orders. 



In 1901 Borner first divided the Collembola into two suborders, the elongate 

 forms with relatively discrete abdominal segments being the Arthropleona 

 and the globular forms with the abdominal segments much fused being the 

 Symphypleona. 



During the first quarter of the present century Handlirsch, Borner, and 

 Crampton vied with each other in shuffling arrangements and names for the 

 grouping of the primitively wingless hexapods, but little new evidence was pro- 

 duced or brought to bear on the problem. 



The Protura were not recognized in print until 1907, when Silvestri de- 

 scribed the first genus and species and placed them in a new order. While small, 

 most Protura are easily seen with the naked eye. They occur abundantly in 

 soil in all or nearly all temperate and tropical regions of the world. It is most 

 curious that the Protura passed unnoticed for such a long time. Silvestri's ex- 

 citing paper was followed soon by numerous others by several authors. In 1908 

 Berlese described the internal anatomy, and in 1909 he published a superb mono- 

 graph of all aspects of the group. At that time, only two years after the order 

 was named, he knew ten species. 



Tillj^ard (1931) introduced a new theory of insect ancestry, in characteristi- 

 cally logical style. Stressing the site of the gonopore, which Packard had em- 

 phasized long before, Tillyard first divided the insects and close allies into two 

 great groups, the Progoneata (Symphyla, Pauropoda, Diplopoda) and the Opis- 

 thogoneata (Chilopoda and hexapods). The hexapod line produced the Collem- 

 bola, then Protura, then in successive steps the Projapygidae, Campodeidae, and 

 the first "Ectotrophica." The last gave rise to two surviving lines, the Machili- 

 dae and the Lepismatidae, with the latter producing the Pterygota. Imms (1936, 

 1939, 1947) convincingly discounted the gonopore character and reasserted the 

 old view that the Symphyla and Entotrophi (for which he used Borner's name, 

 Diplura) are very closely related. 



In 1940, in the first of a series of brilliant contributions, Tiegs showed that 

 the gonopore of the Symphyla is not primarily anterior. Pie wrote: "The first 

 instar larva presents the appearance of a potentially opisthogoneate myriapod. 

 The secondary genital opening on the fourth segment develops in a later instar." 

 He showed (1947) that the Pauropoda have a similar, but not necessarily mono- 

 phyletic, secondary development of the cephalad gonoducts. Tiegs investigated 

 the ontogenetic development of one or a few members each, of Sjonphyla (1940, 

 1945), Pauropoda (1947), Collembola (1942a), and Entotrophi (1942b) in more 

 or less detail. He discovered in eggs of Symphyla, Collembola, and Entotrophi 

 a remarkable "dorsal organ" which produces a cagelike net of long tendrils grow- 

 ing out around the embryo. Such an organ is not known in Thysanura, the 

 Pterygota, or any other arthropod. Its complexity and yet striking similarity 

 among the three groups having it suggest strongly a common ancestor not shared 

 with other myriapods and a deep separation from the Thysanura and Pterygota, 

 in both of which the amnion and serosa are present and may have displaced 

 the "dorsal organ." I am proposing that this unique structure be called Tiegs' 

 Organ in honor of the discoverer of its detailed nature and possible phylogenetic 

 significance. 



