B

FIGURE 2.1. Reconstructions of (A) Dasyleptus sp. (Monura); and (B) Rams-delepidion schusteri (Zygentoma). [From J. Kukalova-Peck, 1987, New Carboniferous Diplura, Monura, and Thysanura, the hexapod ground plan, and the role of thoracic side lobes in the origin of wings (Insecta), Can. J. Zool. 65:23272345. By permission of the National Research Council of Canada and the author.]

The early bristletails, like their modern relatives, perhaps fed on algae, lichens, and debris. They escaped from predators by running and jumping, the latter achieved by abrupt flexing of the abdomen.

Monura are unique among Insecta in that they retain cercal legs (Kukalova-Peck, 1985). Other primitive features of this group are the segmented head, fully segmented maxillary and labial palps, lack of differentiation of the thoracic segments, segmented abdominal leglets, the long caudal filament, and the coating of sensory bristles over the body (Kukalova-Peck, 1991). Features they share with the Zygentoma and Pterygota are dicondy-lous mandibles, well-sclerotized thoracic pleura, and the gonangulum, leading Kukalova-Peck (1987) to suggest that the Monura are the sister group of the Zygentoma + Pterygota. Carpenter (1992), however, included the Monura as a suborder of the Microcoryphia. Shear and Kukalova-Peck (1990) suggested, on the basis of their morphology, that monurans probably lived in swamps, climbing on emergent vegetation, and feeding on soft matter. Escape from predators may have occurred, as in the Microcoryphia, by running and jumping.

In contrast to their rapidly running, modern relatives, the early silverfish, for example, the 6-cm-long. Ramsdelepidion schusteri (Figure 2.1B), with their weak legs, probably avoided predators by generally remaining concealed. When exposed, however, the numerous long bristles that covered the abdominal leglets, cerci, and median filament may have provided a highly sensitive, early warning system. Of particular interest in any discussion of apterygote relationships is the extant silverfish Tricholepidion gertschi, discovered in California in 1961. The species is sufficiently different from other recent Zygentoma that it is placed in a separate family Lepidotrichidae, to which some Oligocene fossils also belong. Indeed, Tricholepidion possesses a number of features common to both Microcoryphia and Monura (see Chapter 5, Section 6), leading Sharov (1966) to suggest that the family to which it belongs is closer than any other to the thysanuranlike ancestor of the Pterygota.

3. Evolution of Winged Insects

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