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100 During a period of more than 20 years, beginning in 1917, Tillyard expounded his views on insect phylogeny, stemming from his extensive research into the fossil insects of Australia and North America. Although he made important contributions concerning the origin and relationships of many insect orders, Tillyard's (1918-1920) work on the endopterygotes is particularly well known. In this work he showed that the Hymenoptera and Coleoptera (with the Strepsiptera) form two rather distinct orders, only distantly related to the other endopterygote groups which collectively formed the panorpoid complex. Within the complex, the Mecoptera, Trichoptera, Lepidoptera, Diptera, and Siphonaptera form a well defined group, with the neuropteroid orders clearly distinct from them. In fact, as noted in Chapter 2, Hinton (1958) made a strong case for excluding these orders entirely from the panorpoid complex and placing them closer to the Coleoptera.

While Tillyard was concentrating on the phylogeny of the endopterygotes, his American contemporary, Crampton, was directing his efforts toward solution of the problems of exopterygote relationships, especially the position of the Zoraptera, Embioptera, Grylloblattidae, and Dermaptera. Following his anatomical study on the newly discovered winged zorapteran Zorotypus hubbardi, Crampton (1920) concluded that the Zoraptera were related to the orthopteroid orders, and he placed them in a group (superorder Panisoptera) that also contained the Isoptera, Blattida, and Mantida. However, the following year Crampton revised his views and transferred the Zoraptera to the psocoid (hemipteroid) superorder, after consideration of their wing venation. In 1922 Crampton placed the Zoraptera in the order Psocoptera and suggested that it was from psocoidlike ancestors that the modern hemipteroid orders evolved. Originally, Crampton (1915) had placed the Grylloblattidae in a separate order, Notoptera, in the orthopteroid group. Five years later he concluded that the grylloblattids were closer to the Orthoptera (sensu stricto) than the blattoid groups and made the Grylloblattodea a suborder of the Orthoptera. The modern view is that the grylloblattids are probably survivors of the protothopteran stock from which both the orthopteran and blattoid lines developed. Crampton considered that the closest relatives of the Embioptera were the Plecoptera, placing the two groups in the superorder Panplecoptera. In his early schemes Crampton also placed the Dermaptera in the Panplecoptera. He later changed this view and included them in the orthopteroid superorder, at the same time pointing out that the Diploglossata (Hemimerida) are parasitic Dermaptera.

Almost simultaneously in 1924 Crampton and the Russian paleoentomologist Martynov proposed an apparently natural division of the winged insects on the basis of the ability to flex the wings horizontally over the body when at rest. In the Paleoptera (= Paleopterygota = Archipterygota) are the orders Ephemeroptera and Odonata whose members do not possess a wing-folding mechanism. It must be emphasized, however, that the two orders are only very distantly related through their paleodictyopteran ancestry. The remaining orders, whose members are able to fold their wings over the body, are placed in the Neoptera (= Neopterygota). The latter contains three natural subdivisions, the Polyneoptera (orthopteroid orders), Paraneoptera (hemipteroid orders), and Oligoneoptera (endopterygote orders).

Even recently, vigorous debate has continued over the taxonomic rank of, and nature of the evolutionary relationships among, hexapod groups (see Chapter 1, Section 3.3.1 [apterygotes], and Chapter 2, Section 3.2 [pterygotes] for a fuller discussion). For example, most authors consider the Collembola and Protura to be sister groups and sometimes unite them in the class Ellipura (= Parainsecta). However, the position of the Diplura is less clear; Kristensen (1991) placed them close to the Ellipura principally on the basis

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